973 concepts from 112 boxes

Concepts, A to Z

Every term we define, in one place. Each one links to the box where you can see it working.

#

180-degree rule: Keep the camera on one side of the line between two characters so each stays on the same side of the screen.See it in No. 052: How do screenplays and storyboards work? →
180° rule: Keep the camera on one side of the line between two people so they keep their sides of the screen.See it in No. 065: How does film editing work? →
180° rule: Keep the camera on one side of the line between two actors so they keep their sides of the screen.See it in No. 051: How does a film get made? →
96 thathuvas: The 96 principles that Siddha says make up a person.See it in No. 084: How does Siddha medicine work? →
Δv: The change of velocity in a collision, the best single measure of how much a crash hurts.See it in No. L18: What is momentum? →

A

Above and below the line: The two halves of a budget: creative leads and stars, then crew, sets and equipment.See it in No. 051: How does a film get made? →
ABS: Anti-lock braking: sensors spot a wheel about to lock and a valve eases the brake many times a second.See it in No. 021: How does a motorcycle work? →
Absolute zero: 0 K, the coldest possible temperature. You can get ever closer but never reach it.See it in No. L05: What are the laws of thermodynamics? →
AC and DC: Alternating current reverses direction many times a second; direct current flows one way.See it in No. L27: What are electric current and voltage? →
AC and DC: Alternating current keeps reversing, 50 times a second in India; direct current flows one way, like a battery's.See it in No. 009: How does an inverter or UPS work? →
Acceleration: The rate of change of velocity: a = Δv ÷ Δt, in metres per second per second (m/s²).See it in No. L15: What are speed, velocity and acceleration? →
Acceleration: How quickly velocity changes, in m/s every second. Turning is acceleration too.See it in No. L03: What are Newton's laws of motion? →
Acinus: A grape-like cluster of enzyme-making cells around a tiny central channel.See it in No. 038: How does the pancreas work? →
Acrylamide: A chemical that forms when starchy food is cooked hot and dark. Food agencies advise cooking to golden.See it in No. 017: How does an air fryer work? →
Actin and myosin: The thin and thick filaments; myosin heads pull on actin to make the filaments slide.See it in No. 047: How do your muscles move you? →
Action: The set of levers between a key and its hammer and damper. A grand piano has 88 of them.See it in No. 006: How does a piano work? →
Action potential: A 2-millisecond, all-or-nothing voltage spike that travels along an axon.See it in No. 045: How does the nervous system work? →
Action potential: A spike: a brief, all-or-nothing flip of a neuron's voltage from about −70 mV to about +35 mV.See it in No. 030: How does the brain work? →
Action–reaction pair: Two equal and opposite forces that two objects exert on each other.See it in No. L03: What are Newton's laws of motion? →
Activation function: The squash at the end of a neuron, such as a step, a sigmoid or ReLU.See it in No. 075: How do neural networks work? →
Adaptive immunity: Defence that learns each germ's exact shape and remembers it.See it in No. 049: How does the immune system work? →
Additive colour: Mixing coloured lights: red and green make yellow, and all three make white.See it in No. 014: How does a TV work? →
Address (RIC): A pager's own 21-bit number; its last 3 bits choose which of 8 frames it listens in.See it in No. 072: How does a pager work? →
ADH: Antidiuretic hormone from the pituitary; it tells the kidneys to keep water.See it in No. 044: How does the excretory system work? →
ADH (vasopressin): A pituitary hormone that makes the collecting ducts take back more water.See it in No. 039: How do the kidneys work? →
ADR: Automated dialogue replacement: re-recording lines in a studio to match the lips on screen.See it in No. 057: How do film sound and Foley work? →
Adrenaline and cortisol: Fast and slower stress hormones from the adrenal glands.See it in No. 048: How does the endocrine system work? →
ADSR envelope: Attack, decay, sustain and release: the four settings that shape a note in time.See it in No. 027: How does a synthesizer work? →
Adversarial example: A picture changed on purpose, often invisibly, so that a model gets it wrong.See it in No. 078: How does image recognition work? →
Aerodynamic drag: The push back from the air, ½ × air density × drag coefficient × frontal area × speed². It grows four times when speed doubles.See it in No. 007: How does a car work? →
Agni: The "digestive fire" that, in Ayurveda, turns food into nourishment for the tissues.See it in No. 080: How does Ayurveda work? →
Air reed: A jet of breath that flips in and out across a sharp edge, doing the job a cane reed does in a clarinet.See it in No. 024: How does a flute work? →
Air–fuel ratio: Mass of air per mass of fuel. Petrol burns completely at 14.7 : 1.See it in No. L13: What is combustion? →
Airflow: How much air a fan moves each minute, in cubic metres per minute. A 1200 mm ceiling fan moves about 200 m³/min.See it in No. 004: How does a fan work? →
Akhlat (humours): Unani’s four body fluids: dam (blood), balgham (phlegm), safra (yellow bile) and sauda (black bile).See it in No. 082: How does Unani medicine work? →
ALU: Arithmetic logic unit: the part of the CPU that adds, subtracts and compares.See it in No. 074: How does a computer work? →
Alveoli: The hundreds of millions of tiny air sacs at the ends of the airways, where gases pass between air and blood.See it in No. 029: How do the lungs work? →
Amp-hour: How much charge a battery holds. Volts × amp-hours gives the energy in watt-hours.See it in No. 009: How does an inverter or UPS work? →
Anamorphic lens: A lens that squeezes the picture sideways onto the sensor, stretched back later to a wide 2.39:1.See it in No. 054: How does a cinema camera work? →
Angular momentum: The momentum of spinning, L = I ω, also conserved. Light carries momentum too: p = E ÷ c.See it in No. L18: What is momentum? →
Angular momentum: L = I ω. It stays constant unless a torque acts, which is why skaters spin faster with arms in.See it in No. L17: What is inertia? →
Angular speed (ω): How fast something turns, in radians per second; 60 rpm is 6.28 rad/s. Points further out move faster: v = ω r.See it in No. L25: What keeps things moving in circles? →
Animatic: Storyboard panels cut together in time with rough sound, to test a scene's rhythm.See it in No. 052: How do screenplays and storyboards work? →
Animatronic: A mechanical puppet moved by servos or cables under a lifelike skin.See it in No. 056: How do practical special effects work? →
Anosmia: Loss of the sense of smell.See it in No. 033: How does the nose smell? →
Antagonist pair: Two muscles that pull a joint opposite ways, like biceps and triceps.See it in No. 047: How do your muscles move you? →
Anti-scatter grid: Thin lead strips lined up with the beam that block photons scattered sideways in the body.See it in No. 018: How does an X-ray machine work? →
Antibody: A Y-shaped protein made by plasma cells whose two tips lock onto one antigen.See it in No. 049: How does the immune system work? →
Antigen: A piece of a germ that receptors and antibodies recognise.See it in No. 049: How does the immune system work? →
Apparent depth: How deep water looks. Straight down it is the real depth ÷ n, about three quarters.See it in No. L12: What is Snell's law of refraction? →
Apparent motion: Seeing movement in a series of still images that jump a little each time, made by the brain's motion detectors.See it in No. 051: How does a film get made? →
Apparent weight: The push you feel from a seat or floor. It changes in a curve: lighter at the top of a giant wheel, heavier at the bottom.See it in No. L25: What keeps things moving in circles? →
Archimedes' principle: F = ρ_fluid × V_submerged × g: a body in a fluid is pushed up by the weight of the fluid it pushes aside.See it in No. L23: What are density and buoyancy? →
Armature: The posable metal skeleton inside a puppet, with stiff ball-and-socket joints.See it in No. 059: How does stop-motion animation work? →
Arteriole: The smallest artery, with a muscle ring that acts like a tap on the blood supply.See it in No. 041: How does the circulatory system work? →
Artery and vein: Arteries carry blood away from the heart; veins carry it back. Direction, not colour, decides.See it in No. 041: How does the circulatory system work? →
Artificial neuron: A tiny calculation: inputs times weights, plus a bias, passed through an activation function.See it in No. 075: How do neural networks work? →
Asbab-e-sitta zarooriya: The six essential factors: air, food and drink, movement and rest, emotions, sleep and waking, retention and elimination.See it in No. 082: How does Unani medicine work? →
Atmospheric perspective: Far things look paler, bluer and lower in contrast because of the air in between.See it in No. 053: How does concept art work? →
Atmospheric pressure: The weight of the air above pressing on everything, about 101 kPa at sea level and less as you climb.See it in No. L22: What is pressure? →
ATP: The energy molecule every muscle stroke spends, remade by phosphocreatine, glycolysis and aerobic respiration.See it in No. 047: How do your muscles move you? →
Atrium and ventricle: The heart's upper collecting chambers (atria) and lower pumping chambers (ventricles).See it in No. 028: How does the heart work? →
Attention: Each token scores every other token and takes a weighted mix of their information.See it in No. 079: How do AI video models work? →
Attention: Each token scores the others with query-key dot products and takes a weighted mix of their values.See it in No. 076: How do large language models (LLMs) work? →
Attenuation: The weakening of the beam as matter absorbs or scatters photons, following I = I₀·e^(−μx).See it in No. 018: How does an X-ray machine work? →
Average density: Total mass over the total volume enclosed, air spaces included. It decides whether a ship floats.See it in No. L23: What are density and buoyancy? →
Average shot length: Running time divided by the number of shots.See it in No. 065: How does film editing work? →
Avogadro's number: 6.022 × 10²³, the number of particles in one mole of a substance.See it in No. 081: How does homeopathy work? →
Axial and appendicular skeleton: The 80 bones down the body's middle, and the 126 bones of the girdles and limbs.See it in No. 046: How does the skeletal system work? →
Ayurveda: India's classical system of medicine, "knowledge of life", set out in Sanskrit texts.See it in No. 080: How does Ayurveda work? →
AYUSH: India’s ministry for Ayurveda, Yoga, Unani, Siddha, Sowa-Rigpa and Homoeopathy, set up in 2014; CCRUM is its Unani research council.See it in No. 082: How does Unani medicine work? →

B

Back light: A light behind the subject that draws a bright rim on hair and shoulders.See it in No. 055: How does film lighting work? →
Back-voltage: The voltage a spinning armature makes against the mains. The faster it spins, the less current can flow.See it in No. 016: How does a mixer grinder work? →
Backlight: A light behind the subject shining towards the lens; it makes rain, smoke and hair glow.See it in No. 056: How do practical special effects work? →
Backlight: The LEDs and films behind an LCD panel that make the light it shapes into a picture.See it in No. 014: How does a TV work? →
Backpropagation: Passing the output error backwards through the network to find every weight's gradient.See it in No. 075: How do neural networks work? →
Balanced equation: A reaction written with the same number of each kind of atom on both sides.See it in No. L13: What is combustion? →
Banking: Tilting a road or track inwards so the road's own push helps turn you.See it in No. L25: What keeps things moving in circles? →
Barometer: An instrument that measures air pressure, first a column of mercury about 760 mm tall (Torricelli, 1643).See it in No. L22: What is pressure? →
Baroreceptor: A stretch sensor in the neck and aorta that helps the brain keep blood pressure steady.See it in No. 041: How does the circulatory system work? →
Basilar membrane: The strip in the cochlea that vibrates most at a different place for each pitch.See it in No. 032: How does the ear hear? →
Batching: Serving many users' next tokens with one read of the weights.See it in No. 077: How does AI inference work? →
BCD: Binary-coded decimal: each decimal digit kept in its own 4 bits.See it in No. 073: How does a calculator work? →
BCH code: An error-correcting code; POCSAG's BCH(31,21) with parity fixes up to 2 wrong bits per codeword.See it in No. 072: How does a pager work? →
Beats: The slow swell and fade when two nearly equal frequencies sound together. The beat rate is their difference.See it in No. 026: How does a harmonium work? →
Bessel function: The curve that describes how a round skin bends; its zeros set a plain drumhead's mode frequencies.See it in No. 025: How does a tabla work? →
Bile: A liquid from the liver, stored in the gallbladder, that breaks fat into tiny droplets.See it in No. 043: How does the digestive system work? →
Bile: A yellow-green liquid made by the liver. Its bile salts break fat into tiny droplets so it can be digested.See it in No. 037: How does the liver work? →
Bilirubin: Yellow pigment from old red blood cells, sent out in bile; it colours stool and urine.See it in No. 044: How does the excretory system work? →
BLDC motor: A brushless motor with magnets on the rotor and electronics that switch the coils. Fans with one use less than half the power.See it in No. 004: How does a fan work? →
Blend mode: The maths used to merge two layers: over, add, multiply, screen and others.See it in No. 062: How do visual effects (VFX) work? →
Blend shape: A saved face expression mixed in by a weight from 0 to 1.See it in No. 063: How does motion capture work? →
Blend shape: A sculpted variation of a mesh, like a smile, mixed in with a slider.See it in No. 060: How does 3D animation work? →
Blind spot: The optic disc, where the optic nerve leaves the eye and there are no rods or cones.See it in No. 031: How does the eye see? →
Blinding: Keeping patients and assessors from knowing who got which treatment.See it in No. 083: How does modern medicine (allopathy) work? →
Blood pressure: The push of blood on artery walls, given as systolic over diastolic in mmHg, around 120/80 in a healthy adult.See it in No. 028: How does the heart work? →
BNYS: Bachelor of Naturopathy and Yogic Sciences, India's 5½-year degree in naturopathy and yoga.See it in No. 085: How does naturopathy work? →
Boehm system: Theobald Boehm's 1847 flute: large holes placed for good tuning, closed by linked, padded keys.See it in No. 024: How does a flute work? →
Bohr effect: Heat, acid and carbon dioxide make haemoglobin release oxygen more easily, right where muscles are working.See it in No. 042: How does the respiratory system work? →
Boiling point: The temperature where water’s vapour pressure matches the pressure around it: lower up mountains, higher in a pressure cooker.See it in No. L22: What is pressure? →
Bol: A spoken syllable for a tabla stroke, like na, tin, tun, ge, ke or dha.See it in No. 025: How does a tabla work? →
BOLD: Blood-oxygen-level-dependent signal: the change fMRI uses to see which brain areas are working.See it in No. 020: How does an MRI machine work? →
Boundary layer: The thin skin of slow air next to a surface that slows heat flow. Fast air thins it.See it in No. 017: How does an air fryer work? →
Boyle's law: At a fixed temperature, a gas's pressure falls as its volume grows. It is why a bigger chest pulls air in.See it in No. 029: How do the lungs work? →
Boyle's law: At steady temperature, pressure × volume stays the same.See it in No. L06: What are the gas laws? →
BRDF: A surface's rule for how light arriving from one direction leaves in each other direction.See it in No. 061: How does CGI rendering work? →
Breakaway: A prop built to break easily and safely, like sugar glass or a balsa chair.See it in No. 056: How do practical special effects work? →
Bremsstrahlung: "Braking radiation": X-rays made when fast electrons swerve and slow down near atomic nuclei.See it in No. 018: How does an X-ray machine work? →
Bunsen principle: A fast jet of gas drags the surrounding air into a tube with it, premixing fuel and air.See it in No. 011: How do a kitchen chimney and gas hob work? →
Buoyant force: The upward push of a fluid, equal to the weight of the fluid displaced.See it in No. L23: What are density and buoyancy? →
Byte: Eight bits, which can stand for any number from 0 to 255.See it in No. 074: How does a computer work? →
Byte-pair encoding: Building a token vocabulary by merging the most frequent neighbouring pair, again and again.See it in No. 076: How do large language models (LLMs) work? →

C

Cache: A small, fast memory that keeps copies of recently used data close to the CPU.See it in No. 074: How does a computer work? →
Cadence: How fast you pedal, in revolutions per minute.See it in No. 003: How does a bicycle work? →
Cam: A shaped part that turns a push into a rotation, like the one inside a click pen.See it in No. 002: How does a pen work? →
Camera obscura: Latin for 'dark room': a sealed space with a small hole that projects an upside-down image of the outside world.See it in No. 001: How does a camera actually see? →
Camera tracking: Measuring where the camera is and where it points, many times a second.See it in No. 064: How does virtual production work? →
Cantilever: A beam fixed at one end and free at the other, like a diving board or a reed tongue.See it in No. 026: How does a harmonium work? →
Capacitor: An electrical part that, in a fan motor, delays the current in one coil so the magnetic field rotates.See it in No. 004: How does a fan work? →
Capillary: The tiniest vessel, about one red cell wide, where oxygen, food and waste are swapped.See it in No. 041: How does the circulatory system work? →
Capillary action: A liquid rising in a narrow gap because it clings to the walls more than to itself.See it in No. 002: How does a pen work? →
Capillary tube: A long, pin-thin copper tube whose friction drops the refrigerant's pressure so it can boil cold.See it in No. 015: How does a refrigerator work? →
Carbon monoxide: A poisonous, invisible gas made when fuel burns without enough oxygen.See it in No. 011: How do a kitchen chimney and gas hob work? →
Carbon transmitter: A microphone whose carbon granules change their resistance when sound squeezes them.See it in No. 070: How does a telephone work? →
Cardiac cycle: One heartbeat: diastole, when the ventricles relax and fill, then systole, when they contract and eject blood.See it in No. 028: How does the heart work? →
Cardiac output: Heart rate times stroke volume: about 5 litres a minute at rest, 15 to 20 in hard exercise.See it in No. 028: How does the heart work? →
Carnot limit: The best efficiency any heat engine can have: 1 − T_cold ÷ T_hot, in kelvin.See it in No. L05: What are the laws of thermodynamics? →
Cel: A clear sheet with the character inked on the front and painted on the back.See it in No. 058: How does 2D animation work? →
Cell: The patch of ground served by one tower antenna; many cells tile a city like a honeycomb.See it in No. 071: How does a mobile phone work? →
Central nervous system: The brain and spinal cord.See it in No. 045: How does the nervous system work? →
Centre of gravity: The point where an object's whole weight seems to act. It must stay above the base for the object to stand.See it in No. L24: What is torque? →
Centrifugal force: The outward force you seem to feel in a turning frame. It is inertia, not a push from anything.See it in No. L25: What keeps things moving in circles? →
Centripetal acceleration: The acceleration towards the centre of a circle, a = v² ÷ r = ω² r, in m/s².See it in No. L25: What keeps things moving in circles? →
Centripetal acceleration: The acceleration towards the centre needed to move in a circle: a = v² ÷ r.See it in No. L15: What are speed, velocity and acceleration? →
Centripetal acceleration: The push towards the centre that keeps something moving in a circle: a = ω²r.See it in No. 012: How does a washing machine work? →
Centripetal force: The inward pull or push that keeps something moving in a circle: F = m v² ÷ r.See it in No. L25: What keeps things moving in circles? →
Centripetal force: The push towards the centre that keeps anything moving in a circle: m v² ÷ r.See it in No. L03: What are Newton's laws of motion? →
Charles's law: At steady pressure, volume grows in step with kelvin temperature.See it in No. L06: What are the gas laws? →
Chemoreceptor: A sensor for chemicals in the blood. Breathing ones track carbon dioxide, acid and oxygen.See it in No. 042: How does the respiratory system work? →
Chroma key: Making one colour, usually green or blue, transparent so another picture shows through.See it in No. 062: How do visual effects (VFX) work? →
Chyme: The thick, acidic soup of part-digested food that leaves the stomach.See it in No. 043: How does the digestive system work? →
Chyme: The soupy mix of part-digested food and stomach juice that enters the small intestine.See it in No. 036: How do the intestines work? →
Chyme: The soupy mix of partly digested food and gastric juice that leaves the stomach.See it in No. 035: How does the stomach digest food? →
Cirrhosis: Severe, lasting scarring of the liver after years of damage, which stops it working well.See it in No. 037: How does the liver work? →
Clapperboard: A slate with sticks that clap on one frame, so the separately recorded sound can be synced.See it in No. 051: How does a film get made? →
Class 3 lever: A lever with the effort between pivot and load, like the forearm: it trades force for speed.See it in No. 046: How does the skeletal system work? →
Classifier-free guidance: Pushing each step further towards the prompt by mixing guesses made with and without it.See it in No. 079: How do AI video models work? →
Clipping: When a signal is pushed past an amplifier's limit, so the tops of the wave are cut off and new harmonics appear.See it in No. 023: How does a guitar work? →
Clock speed: How many steps a CPU takes each second; 4 GHz is 4 billion ticks a second.See it in No. 074: How does a computer work? →
Closed and open systems: A closed system lets no matter in or out; an open one, like a candle in a room, swaps matter with its surroundings.See it in No. L13: What is combustion? →
Clotting cascade: A chain of blood proteins, each switching on many of the next, that ends in fibrin threads.See it in No. 041: How does the circulatory system work? →
CMYK: Cyan, magenta, yellow and key (black): the four inks most printers use, mixing by taking light away.See it in No. 068: How does a printer work? →
CNN: A convolutional neural network: layers of learned filters and pooling, then an answer.See it in No. 078: How does image recognition work? →
Cochlea: The snail-shaped, fluid-filled inner ear, 2.75 turns, that turns sound into nerve signals.See it in No. 032: How does the ear hear? →
Codec: The method that compresses video for sending and decompresses it in your TV, such as HEVC or AV1.See it in No. 014: How does a TV work? →
Codeword: 32 bits: 21 information bits, 10 BCH check bits and 1 parity bit.See it in No. 072: How does a pager work? →
Coefficient of friction (μ): Friction divided by normal force. It has no unit and depends on the two surfaces.See it in No. L20: What is friction? →
Colour grade: Adjusting the colour and brightness of every shot so they match and set a mood.See it in No. 051: How does a film get made? →
Colour temperature: The colour of a light in kelvin: tungsten 3,200 K, daylight 5,600 K.See it in No. 055: How does film lighting work? →
Combinatorial code: Each smell is recognised by the combination of receptor types it activates, like a barcode.See it in No. 033: How does the nose smell? →
Combustion: A fast reaction of a fuel with oxygen that gives out heat and light.See it in No. L13: What is combustion? →
Commutator: A ring of copper bars on the motor shaft. The carbon brushes touch different bars as it turns, switching the armature coils.See it in No. 016: How does a mixer grinder work? →
Compact and spongy bone: The dense outer shell of a bone, and the lattice of struts inside its ends.See it in No. 046: How does the skeletal system work? →
Compiler: A program that translates source code into machine code.See it in No. 074: How does a computer work? →
Compressibility: How much a fluid shrinks under pressure; tiny for liquids, large for gases.See it in No. L08: What is Pascal's principle? →
Compression ratio: How much the mixture is squeezed before it burns: the cylinder's largest volume divided by its smallest, about 10 to 1 in a petrol engine.See it in No. 007: How does a car work? →
Compressor: The pump at the bottom back of the fridge that squeezes refrigerant gas and drives it round the loop.See it in No. 015: How does a refrigerator work? →
Compressor: The pump that squeezes refrigerant gas, raising its pressure and temperature and driving it round the loop.See it in No. 005: How does an air conditioner work? →
Concept art: Drawings and paintings that design how a film's world, characters and objects look before they are built.See it in No. 053: How does concept art work? →
Condenser: Hot pipes, in the side walls or on the back, where the refrigerant gives its heat to the kitchen and turns liquid.See it in No. 015: How does a refrigerator work? →
Condenser: The hot outdoor coil, where the refrigerant condenses back to liquid and gives its heat to the outside air.See it in No. 005: How does an air conditioner work? →
Conduction: Heat passed from atom to atom through touch. Fourier's law: q = k A ΔT ÷ d.See it in No. L10: How does heat move? →
Confidence interval: The range of true effects that fit a trial's data, usually at 95%.See it in No. 083: How does modern medicine (allopathy) work? →
Consent: Freely saying yes. Everyone has the right to say no about their own body.See it in No. 050: How does the reproductive system work? →
Conservation of mass: In a closed system, the total mass stays the same through any chemical reaction.See it in No. L13: What is combustion? →
Conservation of momentum: With no outside push, the total momentum of colliding or exploding objects stays the same.See it in No. L18: What is momentum? →
Contact image sensor: A page-wide bar of LEDs, lenses and light sensors that reads one line of the page at a time.See it in No. 069: How does a fax machine work? →
Content credentials: A signed record attached to a file saying how it was made and edited (the C2PA standard).See it in No. 079: How do AI video models work? →
Continuity: Area × speed is the same at every section of a pipe, so flow speeds up where it narrows.See it in No. L02: What is Bernoulli's principle? →
Continuity editing: Cutting so that time, space and action seem to flow on without a break.See it in No. 065: How does film editing work? →
Contrast: How different neighbouring parts of the picture look, such as bone against muscle.See it in No. 018: How does an X-ray machine work? →
Convection: Warm, lighter fluid rising and cool, denser fluid sinking, carrying heat with it.See it in No. L23: What are density and buoyancy? →
Convection: Heat carried by a moving fluid, such as air or oil. A fan makes it forced convection.See it in No. 017: How does an air fryer work? →
Convection: Heat carried by a moving fluid: natural when warm fluid rises by itself, forced when a fan or pump drives it. q = h A ΔT.See it in No. L10: How does heat move? →
Conventional current: Current drawn flowing from + to −. Electrons, being negative, drift from − to +.See it in No. L27: What are electric current and voltage? →
Convolution: Sliding a small grid of weights over a picture and adding up at every spot.See it in No. 078: How does image recognition work? →
Convolution: Sliding a small grid of weights over an image to find patterns such as edges.See it in No. 075: How do neural networks work? →
Cooling fins: Thin metal plates around an air-cooled cylinder that multiply the surface the air can take heat from.See it in No. 021: How does a motorcycle work? →
COP: Coefficient of performance: heat moved divided by electricity used. About 3.5 for a good AC on a hot day.See it in No. 005: How does an air conditioner work? →
CORDIC: A way to compute sin and cos by turning in shrinking steps, using only shifts and adds.See it in No. 073: How does a calculator work? →
Coriolis effect: In a turning frame, moving things seem to curve sideways. On Earth it steers winds and cyclones.See it in No. L25: What keeps things moving in circles? →
Cornea: The clear dome at the front of the eye, which does about two-thirds of the focusing.See it in No. 031: How does the eye see? →
Cortex: The folded outer 2 to 4 mm of grey matter, where most thinking, sensing and planning happens.See it in No. 030: How does the brain work? →
Coulomb: The unit of charge: about 6.24 × 10¹⁸ electrons. One amp is one coulomb per second.See it in No. L27: What are electric current and voltage? →
Countersteering: Briefly steering away from a turn to make the bike lean into it.See it in No. 021: How does a motorcycle work? →
Counterweight: A weight on the other end of a line that balances or lifts a performer.See it in No. 056: How do practical special effects work? →
Counterweight: Concrete bolted to the tub so an uneven load moves it less.See it in No. 012: How does a washing machine work? →
Coverage: Filming one scene from several set-ups, such as a master, over-the-shoulders and close-ups, so the editor has choices.See it in No. 052: How do screenplays and storyboards work? →
CPU: The processor: the chip that fetches, decodes and executes a program's instructions.See it in No. 074: How does a computer work? →
Critical angle: Going into a faster material, the angle beyond which no light gets out: sin θc = n₂ ÷ n₁. About 42° for glass to air.See it in No. L12: What is Snell's law of refraction? →
Critical point: Above it, no squeeze can turn a gas into a liquid. For CO₂: 31 °C and 74 bar.See it in No. L06: What are the gas laws? →
Critical speed: The drum speed at which clothes stay pinned to the wall all the way round: ω = √(g/r), about 60 rpm for a 50 cm drum.See it in No. 012: How does a washing machine work? →
Crop factor: How much smaller a sensor is than 36 × 24 mm full frame. Super 35 is about 1.4 to 1.5×.See it in No. 054: How does a cinema camera work? →
Cross-bridge cycle: Grab, pull, release, re-cock: one ATP per stroke for each myosin head.See it in No. 047: How do your muscles move you? →
Curie point: The temperature above which a ferromagnet loses its magnetism: 770 °C for iron.See it in No. L26: What is magnetism? →
Current: How much electric charge flows past a point each second, measured in amps (A).See it in No. L01: What is Ohm's law? →
Curved spacetime: Einstein's picture of gravity: mass bends space and time, and things follow the curves.See it in No. L21: What is gravity? →
Cut: An instant change from one shot to the next.See it in No. 065: How does film editing work? →

D

Damper: A felt pad resting on a note's strings that stops them ringing when the key is let go.See it in No. 006: How does a piano work? →
Danger zone: About 5 to 60 °C, the temperatures where bacteria in food multiply fastest.See it in No. 015: How does a refrigerator work? →
Data augmentation: Randomly turning, flipping, re-lighting and adding noise to training pictures so a network can't memorise them.See it in No. 078: How does image recognition work? →
Dayan and bayan: The right-hand wooden drum with a clear pitch, and the left-hand metal bass drum.See it in No. 025: How does a tabla work? →
DCP and KDM: The Digital Cinema Package of encrypted files, and the Key Delivery Message that unlocks it for one server and time window.See it in No. 067: How does a cinema projector work? →
Debouncing: Ignoring a switch until its flickering contact has settled.See it in No. 073: How does a calculator work? →
Decibel: A log scale for sound level: +10 dB is ten times the power and sounds about twice as loud.See it in No. 032: How does the ear hear? →
Deflagration: Burning that spreads slower than sound, like a film fireball: bright but gentle.See it in No. 056: How do practical special effects work? →
Denoiser: The trained network that guesses the clean picture from a noisy one.See it in No. 079: How do AI video models work? →
Density (ρ): Mass divided by volume, in kg/m³ or g/cm³. Water is 1,000 kg/m³.See it in No. L23: What are density and buoyancy? →
Depth of field: The zone from near to far that looks sharp. Long lenses, wide apertures and big sensors make it thin.See it in No. 054: How does a cinema camera work? →
Depth of field: The distance range that looks acceptably sharp. Wide apertures, long lenses and close subjects make it shallow.See it in No. 001: How does a camera actually see? →
Dermis: The tough middle layer of collagen and elastin, holding vessels, nerves, glands and hair roots.See it in No. 040: How does the skin work? →
Detrusor: The muscle wall of the bladder: relaxed while filling, squeezing to empty.See it in No. 044: How does the excretory system work? →
Dew point: The temperature at which air is saturated with water vapour. Cool it further and water condenses out.See it in No. 005: How does an air conditioner work? →
Dialysis: Cleaning the blood with a machine filter or the belly's own lining when the kidneys fail.See it in No. 039: How do the kidneys work? →
Diaphragm: The dome-shaped muscle under the lungs. It flattens to pull air in and is the main muscle of breathing.See it in No. 042: How does the respiratory system work? →
Diaphragm: The dome-shaped muscle under the lungs. It flattens to pull air in and is the main muscle of breathing.See it in No. 029: How do the lungs work? →
Dielectric heating: Heating by a rapidly flipping electric field that twists polar molecules such as water against their neighbours.See it in No. 008: How does a microwave oven work? →
Differential: A set of gears that splits the drive between two wheels and lets them turn at different speeds in a corner.See it in No. 007: How does a car work? →
Diffusion: The spreading of molecules from where there are more to where there are fewer, by random jostling. A cloud grows as √(2Dt).See it in No. L14: What are osmosis and diffusion? →
Diffusion model: A generator that starts from random noise and removes it step by step to reveal a picture or video.See it in No. 079: How do AI video models work? →
Dioptre: A unit of focusing power: 1 divided by the focal length in metres. The whole eye has about 60.See it in No. 031: How does the eye see? →
Director of photography: The cinematographer, who designs the lighting and camera work.See it in No. 051: How does a film get made? →
Dispersion: n changing with colour, so violet bends a little more than red. It makes prisms and rainbows.See it in No. L12: What is Snell's law of refraction? →
Displacement: The fluid an object pushes aside; for a ship, the mass of water equal to the ship's own mass.See it in No. L23: What are density and buoyancy? →
Displacement: How far, and in which direction, you are from where you started.See it in No. L15: What are speed, velocity and acceleration? →
DIT: The digital imaging technician, who copies, checks and backs up footage on set and makes proxies.See it in No. 054: How does a cinema camera work? →
DMD: A digital micromirror device: a chip of millions of hinged mirrors, one per pixel, tilting on or off.See it in No. 067: How does a cinema projector work? →
Dolby Atmos: Cinema sound with ceiling speakers where each sound is an object with a 3D position.See it in No. 057: How do film sound and Foley work? →
Dosha: One of three functional principles, vata, pitta and kapha, whose balance Ayurveda equates with health.See it in No. 080: How does Ayurveda work? →
Double circulation: Blood passes through the heart twice per lap: once for the lungs, once for the body.See it in No. 041: How does the circulatory system work? →
dpi: Dots per inch: how many dots a printer can place along one inch.See it in No. 068: How does a printer work? →
Drift speed: The slow average speed of electrons along a wire, well under a millimetre per second.See it in No. L01: What is Ohm's law? →
Drum and tub: The perforated steel drum holds and spins the clothes; the still outer tub around it holds the water.See it in No. 012: How does a washing machine work? →
DTMF: Dual-tone multi-frequency: each key sends one low and one high tone at the same time.See it in No. 070: How does a telephone work? →
Dual-energy CT: Scanning at two X-ray energies to tell materials apart by how their attenuation changes with energy.See it in No. 019: How does a CT scanner work? →
Duodenum, jejunum, ileum: The three parts of the small intestine, in order from the stomach.See it in No. 036: How do the intestines work? →
Duty cycle: The fraction of time a part is switched on; a pager's receiver is on about a fifth of the time.See it in No. 072: How does a pager work? →
Duty cycle: The share of time a heater is switched on.See it in No. 017: How does an air fryer work? →
Duty cycle: How long a machine may run before it must rest, such as 30 seconds on and one minute off.See it in No. 016: How does a mixer grinder work? →
Duty cycle: The share of time the compressor runs, set by the thermostat's cut-in and cut-out temperatures.See it in No. 015: How does a refrigerator work? →
Duty cycle: The share of time something is switched on. 50% power on a classic oven means on half the time.See it in No. 008: How does a microwave oven work? →
Dynamic pressure: ½ρv², the energy of motion in each cubic metre of moving fluid.See it in No. L02: What is Bernoulli's principle? →

E

E = mc²: Mass is a very concentrated form of energy: 1 gram is worth about 90 trillion joules.See it in No. L04: What is conservation of energy? →
Eardrum: The thin tympanic membrane at the end of the ear canal that vibrates with sound.See it in No. 032: How does the ear hear? →
Eccrine gland: The common sweat gland, a coiled tube opening straight onto the skin.See it in No. 040: How does the skin work? →
ECG: Electrocardiogram: a recording of the heart's electrical waves, labelled P, QRS and T.See it in No. 028: How does the heart work? →
Eddy currents: Swirling currents induced inside solid metal by a changing field; they heat it and brake motion.See it in No. L07: What is Faraday's law of induction? →
Edge loop: A ring of connected edges, laid where the model needs to move or bend.See it in No. 060: How does 3D animation work? →
Efficiency: Useful work out ÷ energy in. The rest is lost, mostly as heat.See it in No. L19: What are work and power? →
Efficiency: Useful energy out ÷ energy in. The rest isn't lost, just not useful: usually heat.See it in No. L04: What is conservation of energy? →
Ejection fraction: The share of the blood in a full ventricle that one beat pumps out, normally about 55 to 70%.See it in No. 028: How does the heart work? →
Elastic and inelastic: Elastic collisions keep kinetic energy too; inelastic ones turn some of it into heat, sound and dents.See it in No. L18: What is momentum? →
Elastic energy: Energy stored in a stretched spring: ½ k x², the area under the force–extension line.See it in No. L09: What is Hooke's law? →
Elastic limit: The largest stretch or stress after which a material still springs fully back.See it in No. L09: What is Hooke's law? →
Electric current: The rate at which charge flows past a point, I = Q ÷ t, measured in amperes (A).See it in No. L27: What are electric current and voltage? →
Electromagnet: A coil, usually round an iron core, that is a magnet only while current flows: B = μ₀ n I inside.See it in No. L26: What is magnetism? →
Electromagnetic induction: Making a voltage in a conductor by changing the magnetic flux through it.See it in No. L07: What is Faraday's law of induction? →
Electromagnetic wave: Linked electric and magnetic fields travelling at c = 299,792 km/s, even through empty space.See it in No. L11: What are waves and resonance? →
Embedding: A learned list of numbers that places a token in a space where similar meanings sit close together.See it in No. 076: How do large language models (LLMs) work? →
Embouchure hole: The hole a flute player blows across. The breath splits on its far edge.See it in No. 024: How does a flute work? →
EMF: Electromotive force: the voltage induced in a coil, even with no battery.See it in No. L07: What is Faraday's law of induction? →
Emissivity (ε): How well a surface radiates compared with a perfect black body: about 0.9 for black iron, 0.03 for silvered glass.See it in No. L10: How does heat move? →
End correction: The extra length a pipe seems to have because the vibrating air pokes a little way out of each open end.See it in No. 024: How does a flute work? →
Endocrine gland: A gland with no duct that releases its hormones straight into the blood.See it in No. 048: How does the endocrine system work? →
Energy: The ability to make something happen. Measured in joules, it changes form but is never made or destroyed.See it in No. L04: What is conservation of energy? →
Enteric nervous system: The gut's own network of hundreds of millions of nerve cells, its second brain.See it in No. 043: How does the digestive system work? →
Entropy (S): A count of the ways energy and molecules can be arranged, S = k ln W. Left alone, it only grows.See it in No. L05: What are the laws of thermodynamics? →
Envagai thervu: The eight-fold examination: pulse, touch, tongue, colour, voice, eyes, stools and urine.See it in No. 084: How does Siddha medicine work? →
Enzyme: A protein that speeds up one reaction, like molecular scissors cutting one kind of bond.See it in No. 043: How does the digestive system work? →
Enzyme: A protein that speeds up a reaction, such as cutting a big food molecule into small ones.See it in No. 036: How do the intestines work? →
Enzyme: A protein that speeds up one reaction, like snipping starch or egg into small pieces that wash away.See it in No. 013: How does a dishwasher work? →
Epidermis: The thin outer layer of skin, renewed from its basal layer about every 4 weeks.See it in No. 040: How does the skin work? →
Epiglottis: A leaf-shaped flap that tips over the larynx when you swallow, so food goes down the food pipe.See it in No. 042: How does the respiratory system work? →
Equal temperament: Tuning where every semitone is exactly 100 cents, so every key works but only octaves are pure.See it in No. 026: How does a harmonium work? →
Equal temperament: A tuning where every semitone has the same frequency ratio, 2^(1/12), so twelve of them make an octave.See it in No. 023: How does a guitar work? →
Equal temperament: Tuning in which every semitone is the same ratio, the twelfth root of 2, so twelve make an octave.See it in No. 006: How does a piano work? →
Equations of motion: For steady acceleration: v = u + a t and s = u t + ½ a t².See it in No. L15: What are speed, velocity and acceleration? →
Equivalence principle: Inertial and gravitational mass are the same, so everything falls alike. Tested to about 1 part in 10¹⁵.See it in No. L17: What is inertia? →
Error correction mode: Sending fax data in checked frames and resending only the damaged ones.See it in No. 069: How does a fax machine work? →
Erythropoietin (EPO): A kidney hormone that tells bone marrow to make red blood cells.See it in No. 039: How do the kidneys work? →
Escapement: The jack slipping out from under the hammer just before it hits, so the hammer flies free and bounces off the string.See it in No. 006: How does a piano work? →
Evaporative cooling: Cooling by turning a liquid, like sweat, into vapour. The change soaks up heat from the surface.See it in No. 004: How does a fan work? →
Evaporator: The cold coil in the freezer, where the refrigerant boils at about −25 °C and takes heat from the air.See it in No. 015: How does a refrigerator work? →
Evaporator: The cold indoor coil, where the refrigerant boils and takes heat from the room air.See it in No. 005: How does an air conditioner work? →
Evidence-based medicine: Choosing treatments by the best fair tests, with the doctor's skill and the patient's wishes.See it in No. 083: How does modern medicine (allopathy) work? →
Excretion: Getting rid of waste your own cells made, such as carbon dioxide, urea and extra salt.See it in No. 044: How does the excretory system work? →
Exocrine and endocrine: Exocrine glands send their product down ducts, like digestive juice; endocrine glands release hormones into the blood, like insulin.See it in No. 038: How does the pancreas work? →
Expansion valve: A narrow opening that drops the refrigerant's pressure, so it becomes cold enough to absorb heat.See it in No. 005: How does an air conditioner work? →
Exposure sheet: A chart with one row per frame, listing sounds, actions and drawings.See it in No. 058: How does 2D animation work? →
Extension: How much longer or shorter a spring is than its natural length.See it in No. L09: What is Hooke's law? →

F

f ∝ √T: A stretched skin's frequency grows with the square root of its tension.See it in No. 025: How does a tabla work? →
f-number: Focal length divided by aperture diameter. Smaller numbers mean bigger openings: f/2 lets in 4x the light of f/4.See it in No. 001: How does a camera actually see? →
Fair test: A randomised, blinded, controlled trial that separates a real effect from chance and expectation.See it in No. 084: How does Siddha medicine work? →
Fan laws: For the same fan, airflow grows with speed, pressure with speed squared and power with speed cubed.See it in No. 004: How does a fan work? →
Feature map: The picture of a filter's answers: bright where it found its pattern.See it in No. 078: How does image recognition work? →
Feed: The part under a fountain pen's nib that sends ink out and lets air in.See it in No. 002: How does a pen work? →
Fertilisation: One sperm and one egg joining in the fallopian tube to make a zygote.See it in No. 050: How does the reproductive system work? →
Fill light: A softer, dimmer light that lifts the shadows made by the key.See it in No. 055: How does film lighting work? →
Filtered back-projection: Rebuilding a slice by sharpening each projection with a ramp filter and smearing it back across the image.See it in No. 019: How does a CT scanner work? →
Final drive: The chain and sprockets from the gearbox to the back wheel, about 2.8 : 1 on a 150 cc commuter.See it in No. 021: How does a motorcycle work? →
Fire triangle: Fuel, heat and oxygen: a fire needs all three.See it in No. L13: What is combustion? →
First law of thermodynamics: Energy in = energy out + change in energy stored. It includes heat, and it always balances.See it in No. L04: What is conservation of energy? →
Flame-failure device: A thermocouple and magnet valve that shut off the gas if the flame goes out.See it in No. 011: How do a kitchen chimney and gas hob work? →
Flicker fusion: The flash rate above which light looks steady; at cinema brightness roughly 40 to 50 Hz.See it in No. 067: How does a cinema projector work? →
FLOP: One floating-point operation, such as a single multiply or add.See it in No. 077: How does AI inference work? →
Flow breakers: Ribs on the inside of a jar that stop the load swirling round with the blade.See it in No. 016: How does a mixer grinder work? →
FM synthesis: Making rich sounds by wobbling one sine wave's frequency with another at audio speed.See it in No. 027: How does a synthesizer work? →
Focal length: How far behind a lens parallel rays meet. For a thin lens, 1/f = (n − 1)(1/R₁ − 1/R₂).See it in No. L12: What is Snell's law of refraction? →
Focal length: How strongly a lens bends light, in millimetres. Short = wide view, long = narrow, magnified view.See it in No. 001: How does a camera actually see? →
Foley: Everyday sounds performed live in sync with the picture after filming.See it in No. 057: How do film sound and Foley work? →
Foot-lambert: A unit of screen brightness; the DCI standard is 14 fL, or 48 cd/m².See it in No. 067: How does a cinema projector work? →
Force: A push or a pull on an object, with a size and a direction.See it in No. L16: What is a force? →
Force: A push or a pull, measured in newtons. One newton gives 1 kg an extra 1 m/s every second.See it in No. L03: What are Newton's laws of motion? →
Four-stroke: An engine cycle of intake, compression, power and exhaust, taking two turns of the crankshaft.See it in No. 021: How does a motorcycle work? →
Four-stroke cycle: Intake, compression, power and exhaust: the four piston strokes, over two crank turns, that make each bang in a petrol engine.See it in No. 007: How does a car work? →
Fovea: A tiny pit in the macula, packed with cones, where vision is sharpest.See it in No. 031: How does the eye see? →
Frame: One still picture in a video; cinema shows 24 every second.See it in No. 079: How do AI video models work? →
Frame: One photo in the film; cinema shows 24 a second.See it in No. 059: How does stop-motion animation work? →
Frame: One still picture in a film; cinema shows 24 a second.See it in No. 058: How does 2D animation work? →
Frame: One still picture in a film. Cinema shows 24 every second.See it in No. 051: How does a film get made? →
Frame of reference: The point of view velocities are measured from, such as the platform or the moving train.See it in No. L15: What are speed, velocity and acceleration? →
Frame rate: How many pictures a camera takes each second. Cinema plays at 24 fps.See it in No. 054: How does a cinema camera work? →
Frame rate: How many still pictures a video shows each second.See it in No. 014: How does a TV work? →
Free fall: Moving under gravity alone. Everything falls together, so you feel weightless.See it in No. L21: What is gravity? →
Free fall: Motion under gravity alone. Near the Earth everything accelerates at g = 9.81 m/s².See it in No. L15: What are speed, velocity and acceleration? →
Free reed: A springy tongue that swings freely through a slot, chopping airflow into puffs. Used in harmoniums, accordions, harmonicas and the sheng.See it in No. 026: How does a harmonium work? →
Frequency: How many times a second something vibrates, in hertz (Hz). The low E string vibrates 82.4 times a second.See it in No. 023: How does a guitar work? →
Frequency: How many times a second a wave repeats. Microwave ovens use 2.45 GHz, 2.45 billion times a second.See it in No. 008: How does a microwave oven work? →
Frequency (f): How many waves pass a point each second, in hertz (Hz).See it in No. L11: What are waves and resonance? →
Fresnel effect: Surfaces reflect more light at glancing angles: about 4% straight on for glass, nearly all at grazing angles.See it in No. 061: How does CGI rendering work? →
Friction: The force between touching surfaces that resists them sliding over each other.See it in No. L20: What is friction? →
Friction: The force between surfaces that resists sliding, up to about μ times the force pressing them together.See it in No. L16: What is a force? →
Frost-free: A fridge with a hidden, fan-blown evaporator and an automatic defrost heater.See it in No. 015: How does a refrigerator work? →
FSK: Frequency shift keying: sending 0s and 1s by nudging the radio frequency up or down.See it in No. 072: How does a pager work? →
Full adder: Five gates that add two bits and an incoming carry.See it in No. 073: How does a calculator work? →
Fundamental forces: Strong, electromagnetic, weak and gravity: the four forces behind every push and pull.See it in No. L16: What is a force? →
Fuser: A hot roller and a pressure roller that melt toner into the paper.See it in No. 068: How does a printer work? →

G

g-force: An acceleration measured in multiples of gravity, a ÷ 9.81. A washer drum at 1,200 rpm reaches about 400 g.See it in No. L25: What keeps things moving in circles? →
Gamete: A sex cell, an egg or a sperm, carrying 23 chromosomes.See it in No. 050: How does the reproductive system work? →
Gamut: The range of colours a screen or standard can show.See it in No. 066: How does colour grading work? →
Gantry: The doughnut-shaped part of a CT scanner that holds the spinning X-ray tube and detector.See it in No. 019: How does a CT scanner work? →
Gatta and gajra: The wooden tuning blocks under the straps, and the braided rim that is tapped to fine-tune.See it in No. 025: How does a tabla work? →
Gauge and absolute pressure: Gauges read the pressure above the surrounding air; absolute pressure adds the air’s 1 atm back.See it in No. L22: What is pressure? →
Gear ratio: Teeth on the driven gear ÷ teeth on the driving gear: it multiplies torque and divides speed.See it in No. L24: What is torque? →
Gear ratio: Teeth on the driven gear divided by teeth on the driving gear: how many turns in for one turn out, and how much the torque is multiplied.See it in No. 007: How does a car work? →
Gear ratio: Front teeth divided by rear teeth: how many times the wheel turns for each turn of the pedals.See it in No. 003: How does a bicycle work? →
Generator: A machine that turns motion into electricity by spinning coils and magnets past each other.See it in No. L07: What is Faraday's law of induction? →
Geneva drive: A pin wheel and slotted Maltese cross that turn smooth rotation into quick quarter-turn steps.See it in No. 067: How does a cinema projector work? →
Genlock: A shared timing signal so camera, engine and screens start every frame together.See it in No. 064: How does virtual production work? →
GFR: Glomerular filtration rate: about 125 mL a minute, or 180 litres a day, in a young adult.See it in No. 039: How do the kidneys work? →
Ghrelin: The hunger hormone, made mostly by the stomach, which rises before meals.See it in No. 035: How does the stomach digest food? →
Gimbal: A powered platform that tilts and shakes a set piece such as a car.See it in No. 056: How do practical special effects work? →
Global illumination: Light that reaches a surface after bouncing off other surfaces.See it in No. 061: How does CGI rendering work? →
Glomerulus: A knot of capillaries where blood pressure filters plasma into Bowman's capsule.See it in No. 039: How do the kidneys work? →
Glucagon: The hormone from alpha cells that tells the liver to release glucose when blood sugar falls.See it in No. 038: How does the pancreas work? →
Gluconeogenesis: The liver making new glucose during a fast, once its glycogen store runs low.See it in No. 085: How does naturopathy work? →
Glycogen: A branching chain of glucose units: the liver's store of sugar, up to about 100 g.See it in No. 037: How does the liver work? →
Goitre: A swollen thyroid, most often from too little iodine.See it in No. 048: How does the endocrine system work? →
Gradient coils: Three coils that make the field vary with position, so the scanner can tell where each signal came from.See it in No. 020: How does an MRI machine work? →
Gradient descent: Moving every weight a small step downhill on the loss, again and again.See it in No. 075: How do neural networks work? →
Gravitational constant (G): 6.674 × 10⁻¹¹ N m²/kg²: the pull between two 1 kg masses 1 m apart.See it in No. L21: What is gravity? →
Gravity: The attraction between any two masses. It always pulls and never cancels out.See it in No. L21: What is gravity? →
Growth plate: A disc of cartilage near the end of a child's long bone where it grows longer.See it in No. 046: How does the skeletal system work? →
Gut microbiome: The trillions of bacteria and other microbes living in your gut, mostly in the colon.See it in No. 043: How does the digestive system work? →

H

H-bridge: Four switches that can connect a load either way round to a battery.See it in No. 009: How does an inverter or UPS work? →
Haematocrit: The share of blood made of red cells, about 40–45%.See it in No. 041: How does the circulatory system work? →
Haemoglobin: The iron-containing protein in red blood cells that carries up to four oxygen molecules.See it in No. 042: How does the respiratory system work? →
Haemoglobin: The iron-containing protein in red blood cells that carries up to four oxygen molecules.See it in No. 029: How do the lungs work? →
Hair cells: Sensory cells with bristles (stereocilia) whose bending opens ion channels and triggers nerve signals.See it in No. 032: How does the ear hear? →
Hakim: A practitioner of Unani medicine; in India, trained through the 5½-year BUMS degree.See it in No. 082: How does Unani medicine work? →
Half-holing: Partly covering a finger hole to bend a note down in between, used for komal notes and meend.See it in No. 024: How does a flute work? →
Half-life: The time for the amount of a drug in the blood to fall by half.See it in No. 083: How does modern medicine (allopathy) work? →
Halftone: A pattern of dots that the eye blends into shades of grey or colour.See it in No. 068: How does a printer work? →
Hallucination: A fluent, confident answer that is false or made up.See it in No. 076: How do large language models (LLMs) work? →
Handover: Moving a phone's connection to a stronger cell without dropping the call.See it in No. 071: How does a mobile phone work? →
Handshake: The opening exchange where two fax machines agree on speed and resolution.See it in No. 069: How does a fax machine work? →
Hard and soft light: Small sources make sharp-edged shadows; large sources make gentle ones.See it in No. 055: How does film lighting work? →
Harmonic: A sine wave at a whole-number multiple of a note's frequency. The mix of harmonics sets the tone.See it in No. 027: How does a synthesizer work? →
Harmonic: A vibration of a string in 2, 3, 4… equal parts, at 2, 3, 4… times the main note. The mix of harmonics is the tone.See it in No. 023: How does a guitar work? →
Harmonic series: Overtones at 2, 3, 4, 5 times the lowest frequency, which our ears hear as one clear note.See it in No. 025: How does a tabla work? →
Harmonics: The whole-number multiples of a string's or pipe's lowest frequency: fₙ = n v ÷ 2L.See it in No. L11: What are waves and resonance? →
HbA1c: A blood test showing how much haemoglobin has glucose attached, a guide to average glucose over about three months.See it in No. 038: How does the pancreas work? →
Heart valve: A flap of tissue that lets blood flow only one way. The heart has four: tricuspid, pulmonary, mitral and aortic.See it in No. 028: How does the heart work? →
Heat: Energy in the jiggling of atoms. Friction, electricity and burning all end up here in the end.See it in No. L04: What is conservation of energy? →
Heat (Q): Energy that flows because of a temperature difference, by itself always from hotter to colder.See it in No. L05: What are the laws of thermodynamics? →
Heat-transfer coefficient (h): How many watts flow into each square metre of surface for each degree of temperature difference.See it in No. 017: How does an air fryer work? →
Heating element: A resistance wire packed in powder inside a metal tube. Current through it makes heat: a 2 kW element at 230 V is about 26 Ω.See it in No. 022: How does a water heater (geyser) work? →
Heating value: Heat from burning 1 kg of fuel: about 50 MJ for methane, 46 for LPG, 44 for petrol and 16 to 18 for dry wood.See it in No. L13: What is combustion? →
Heavy metals: Toxic metals such as mercury, lead and arsenic that build up in the body.See it in No. 084: How does Siddha medicine work? →
Helmholtz resonator: A box of air with a hole. The air in the hole bounces on the springy air inside, like blowing across a bottle.See it in No. 023: How does a guitar work? →
Hepatic artery: The artery that brings oxygen-rich blood to the liver, about a quarter of its supply.See it in No. 037: How does the liver work? →
Hepatocyte: The main liver cell, which stores sugar, builds proteins, makes bile and breaks down toxins.See it in No. 037: How does the liver work? →
Herd immunity: When enough people are immune (1 − 1/R0) that a germ can't keep spreading.See it in No. 049: How does the immune system work? →
Hidden layer: Neurons between the inputs and the output, whose lines combine into a bent boundary.See it in No. 075: How do neural networks work? →
Hijama: Cupping: a cup with low pressure inside sucks up the skin; wet cupping also draws a little blood.See it in No. 082: How does Unani medicine work? →
Hippocampus: A curved structure deep in each temporal lobe that is key to forming new memories.See it in No. 030: How does the brain work? →
Homeopathy: A system of medicine founded by Samuel Hahnemann around 1796, built on "like cures like" and highly diluted remedies.See it in No. 081: How does homeopathy work? →
Hook switch: The switch under the cradle that connects the phone to the line when you lift the handset.See it in No. 070: How does a telephone work? →
Hooke's law: The force on a spring is proportional to its extension: F = k × x.See it in No. L09: What is Hooke's law? →
Hormone: A chemical message made by a gland and carried by the blood to cells elsewhere.See it in No. 048: How does the endocrine system work? →
Horsepower: A unit of power equal to 745.7 watts. For an engine, power is torque times rotation speed.See it in No. 007: How does a car work? →
Horsepower (hp): James Watt's unit of power: 33,000 foot-pounds per minute, 745.7 W.See it in No. L19: What are work and power? →
Hounsfield unit: CT's scale of attenuation: HU = 1000 × (μ − μwater) ÷ μwater, so water is 0 and air is −1000.See it in No. 019: How does a CT scanner work? →
Humbucker: A pickup with two coils wound in opposite directions, so hum cancels while the string's signal adds.See it in No. 023: How does a guitar work? →
Hydraulic press: Two pistons of different areas joined by a liquid, multiplying force by the ratio of their areas.See it in No. L08: What is Pascal's principle? →
Hydrostatic paradox: Pressure at the bottom of a liquid depends only on depth, not on the shape or size of the container.See it in No. L22: What is pressure? →
Hydrostatic paradox: The force on the bottom of a vessel depends on the liquid's depth, not on its amount or the vessel's shape.See it in No. L08: What is Pascal's principle? →
Hydrostatic pressure: Pressure in a still liquid from the weight above: P = ρ g h. About 1 bar per 10 m of water.See it in No. L22: What is pressure? →
Hydrostatic pressure: Pressure from a liquid's own weight at depth h: P = ρ g h.See it in No. L08: What is Pascal's principle? →
Hydrotherapy: Treatment with water at different temperatures, such as hip baths, foot baths, packs and steam.See it in No. 085: How does naturopathy work? →
Hypothalamus: The part of the brain that controls the pituitary and links nerves to hormones.See it in No. 048: How does the endocrine system work? →

I

Ideal gas law: PV = nRT: pressure × volume = moles × 8.314 × temperature in kelvin.See it in No. L06: What are the gas laws? →
Impedance matching: Getting vibrations from air into fluid without most of them bouncing back, the middle ear's main job.See it in No. 032: How does the ear hear? →
Impulse: Force × time, F Δt, in N·s. It equals the change of momentum, so a longer stop means a smaller force.See it in No. L18: What is momentum? →
Impulse: Force × time, equal to the change in momentum. A longer stop needs a smaller force.See it in No. L03: What are Newton's laws of motion? →
IMU: A tiny gyroscope, accelerometer and compass that measures how a body part moves.See it in No. 063: How does motion capture work? →
In-between: A drawing that fills the gap between two keys.See it in No. 058: How does 2D animation work? →
In-camera VFX: Capturing the finished effect through the lens on set, instead of adding it later.See it in No. 064: How does virtual production work? →
Incomplete combustion: Burning with too little oxygen, which leaves carbon monoxide and soot.See it in No. L13: What is combustion? →
Induction motor: A motor whose rotor is pushed by currents that the stator's changing magnetic field induces in it.See it in No. 004: How does a fan work? →
Inertia: The tendency of an object to keep still, or keep moving in a straight line at steady speed, until a force acts.See it in No. L17: What is inertia? →
Inertia: The way matter resists any change in its motion. More mass means more inertia.See it in No. L03: What are Newton's laws of motion? →
Inertial balance: A mass on springs whose wobble period, T = 2π √(m ÷ k), measures mass without gravity.See it in No. L17: What is inertia? →
Inertial frame: A point of view that isn't accelerating or spinning, where Newton's laws hold as written.See it in No. L03: What are Newton's laws of motion? →
Inertial impaction: Catching particles by turning the air so sharply that they cannot follow and hit a surface.See it in No. 011: How do a kitchen chimney and gas hob work? →
Inference: Using a trained model, with its weights frozen, to answer a new question.See it in No. 077: How does AI inference work? →
Innate immunity: Fast, general defences you are born with: barriers, inflammation and eater cells.See it in No. 049: How does the immune system work? →
Inner frustum: The patch of wall the camera sees, drawn in full quality from the camera's own position.See it in No. 064: How does virtual production work? →
Insulin: The hormone from beta cells that lets cells take in glucose and tells the liver to store it.See it in No. 038: How does the pancreas work? →
Insulin and glucagon: Islet hormones that lower and raise blood sugar.See it in No. 048: How does the endocrine system work? →
Insulin and glucagon: Hormones from the pancreas: insulin tells the liver to store glucose, glucagon tells it to release glucose.See it in No. 037: How does the liver work? →
Insulin resistance: When cells respond less to insulin, so more is needed to do the same job; central to type 2 diabetes.See it in No. 038: How does the pancreas work? →
Interference: Waves adding where they overlap: reinforcing in step, cancelling half a wave out of step.See it in No. L11: What are waves and resonance? →
Interlock: A switch that only lets the oven run while the door is fully closed.See it in No. 008: How does a microwave oven work? →
Internal energy (U): All the jiggling and spinning energy of the molecules inside something.See it in No. L05: What are the laws of thermodynamics? →
Inverse kinematics: Posing by moving the end of a chain, like a hand, while the computer solves the joint angles.See it in No. 060: How does 3D animation work? →
Inverse square law: Double the distance from a small lamp and the light falls to a quarter.See it in No. 055: How does film lighting work? →
Inverse-square law: Sound power falls with the square of distance: 6 dB less for each doubling.See it in No. 057: How do film sound and Foley work? →
Inverse-square law: A pull that falls with the square of distance: twice as far, a quarter as strong.See it in No. L21: What is gravity? →
Inverter: A circuit that turns DC into AC by switching a battery's connection back and forth.See it in No. 009: How does an inverter or UPS work? →
Ion channel: A protein doorway in a cell’s skin; sour (OTOP1) and salty (ENaC) tastes work through channels.See it in No. 034: How does the tongue taste? →
Ion exchange: Resin beads that trap calcium ions from hard water and release sodium ions instead.See it in No. 013: How does a dishwasher work? →
IoU: Intersection over union: how much two boxes overlap, from 0 to 1.See it in No. 078: How does image recognition work? →
Islet of Langerhans: A tiny cluster of hormone-making cells; about a million are scattered through the pancreas.See it in No. 038: How does the pancreas work? →

J

J-cut and L-cut: Split edits where the sound changes before (J) or after (L) the picture.See it in No. 065: How does film editing work? →
Joule (J): The unit of work and energy: a force of 1 newton acting over 1 metre.See it in No. L19: What are work and power? →
Jump cut: A cut that removes time from one shot, so things seem to leap.See it in No. 065: How does film editing work? →

K

k-space: The grid of raw echo data. A Fourier transform turns it into the image.See it in No. 020: How does an MRI machine work? →
Kelvin: Temperature counted from absolute zero, −273.15 °C. The gas laws only work in kelvin.See it in No. L06: What are the gas laws? →
Ketones: Fuels the liver makes from fat when food is scarce; the brain can use them.See it in No. 085: How does naturopathy work? →
Key light: The main light on a subject, which sets the direction of the shadows.See it in No. 055: How does film lighting work? →
Key matrix: Keys wired where rows cross columns, so a few wires can serve many keys.See it in No. 073: How does a calculator work? →
Key pose: A main drawing that shows an important moment of an action.See it in No. 058: How does 2D animation work? →
Keyframe: A pose set by the animator at a chosen frame; the computer fills in the frames between.See it in No. 060: How does 3D animation work? →
Kilogram: The SI unit of mass, defined since 2019 by fixing the Planck constant at 6.626 070 15 × 10⁻³⁴ J·s.See it in No. L17: What is inertia? →
Kilowatt-hour (kWh): 1,000 W for one hour: 3.6 MJ. One 'unit' on an electricity bill.See it in No. L19: What are work and power? →
Kinetic energy: Energy of motion, ½ × m × v². Double the speed and it becomes four times as big.See it in No. L04: What is conservation of energy? →
Kinetic energy: The energy of motion, equal to half the mass times the speed squared.See it in No. 003: How does a bicycle work? →
Kuleshov effect: Viewers read different feelings into the same face depending on the shot next to it.See it in No. 065: How does film editing work? →
KV cache: Saved keys and values for every earlier token, so they need not be recomputed.See it in No. 077: How does AI inference work? →
kVp: The peak voltage across the X-ray tube. It sets the top photon energy (in keV) and how penetrating the beam is.See it in No. 018: How does an X-ray machine work? →
Kwatha: A decoction: herbs boiled in water and reduced, usually to a quarter.See it in No. 080: How does Ayurveda work? →

L

Lacteal: The lymph vessel inside each villus that takes in fats.See it in No. 036: How do the intestines work? →
Language model: A program that gives the probability of each possible next piece of text.See it in No. 076: How do large language models (LLMs) work? →
Larmor frequency: How fast protons precess: 42.58 MHz for every tesla of field.See it in No. 020: How does an MRI machine work? →
Latent: A compressed version of the video, made by an autoencoder, that the model actually works on.See it in No. 079: How do AI video models work? →
Latent heat: The heat that turning a liquid into vapour takes: about 2,400 J per gram of sweat.See it in No. 040: How does the skin work? →
Latent heat: Energy that turns a liquid into a gas without warming it: 2,257 kJ per kilogram for water at 100 °C.See it in No. 017: How does an air fryer work? →
Latent heat: The energy needed to turn water into vapour: about 2,400 joules per gram.See it in No. 013: How does a dishwasher work? →
Latent heat: Heat that melts or boils something without changing its temperature: 334 J/g for ice, 2,257 J/g for water.See it in No. L10: How does heat move? →
Latent heat: The energy a liquid takes in as it turns to gas, without getting any hotter.See it in No. 005: How does an air conditioner work? →
Layering: Stacking several sounds so they play as one bigger sound.See it in No. 057: How do film sound and Foley work? →
Learning rate: The size of each step; too big and the loss explodes.See it in No. 075: How do neural networks work? →
LED volume: A stage surrounded by LED walls and often a ceiling, showing a live 3D world.See it in No. 064: How does virtual production work? →
Lens and accommodation: The flexible lens behind the pupil changes shape to focus on near things; this is accommodation.See it in No. 031: How does the eye see? →
Lenz's law: The induced current always flows so as to oppose the change that caused it.See it in No. L07: What is Faraday's law of induction? →
Lever arm: The shortest distance from the pivot to the line of a force, r sin θ.See it in No. L24: What is torque? →
LFO: Low-frequency oscillator: a slow wave that wobbles pitch, tone or loudness.See it in No. 027: How does a synthesizer work? →
Lift: The force on a wing from lower pressure above than below, made by turning the air downwards.See it in No. L02: What is Bernoulli's principle? →
Lift, gamma, gain: Controls for the shadows, mid-tones and highlights of a picture.See it in No. 066: How does colour grading work? →
Lifters: Ribs inside the drum that carry clothes up so they can fall back through the water.See it in No. 012: How does a washing machine work? →
Ligament and tendon: A ligament joins bone to bone; a tendon joins muscle to bone.See it in No. 046: How does the skeletal system work? →
Lighting ratio: The lit side of a face compared with the shadow side, such as 2:1 or 8:1.See it in No. 055: How does film lighting work? →
Lip sync: Matching mouth shapes to the sounds of the voice, frame by frame.See it in No. 059: How does stop-motion animation work? →
Liquid crystal: A liquid of rod-shaped molecules that line up together and can be turned by a voltage, used as a light shutter.See it in No. 014: How does a TV work? →
Load line: The Plimsoll mark on a ship's side showing the deepest it may legally float.See it in No. L23: What are density and buoyancy? →
Lobule: A tiny six-sided unit of the liver, 1 to 2 mm across, with portal triads at its corners and a central vein in the middle.See it in No. 037: How does the liver work? →
Local loop: The pair of copper wires between your phone and the exchange.See it in No. 070: How does a telephone work? →
Log: A recording curve that gives each stop of light a similar share of code values, keeping huge dynamic range.See it in No. 066: How does colour grading work? →
Log recording: A flat-looking curve that keeps all the sensor's stops for the colour grade.See it in No. 054: How does a cinema camera work? →
Logic gate: A small circuit that turns input bits into an output bit by a fixed rule, like AND or XOR.See it in No. 073: How does a calculator work? →
Loop of Henle: The hairpin part of the tubule that makes the medulla salty so urine can be concentrated.See it in No. 039: How do the kidneys work? →
Loss: One number that says how wrong the network is on its training examples.See it in No. 075: How do neural networks work? →
Lower oesophageal sphincter: The ring of muscle that lets food into the stomach and keeps acid out of the oesophagus.See it in No. 035: How does the stomach digest food? →
Lowpass filter: A filter that passes low frequencies and cuts high ones, above its cutoff.See it in No. 027: How does a synthesizer work? →
LPG: Liquefied petroleum gas: mostly propane and butane, stored as a liquid under pressure in a cylinder.See it in No. 011: How do a kitchen chimney and gas hob work? →
Lubrication: A film of oil or grease that keeps surfaces apart and cuts friction.See it in No. L20: What is friction? →
LUFS: A measure of how loud a mix feels, used to set loudness targets.See it in No. 057: How do film sound and Foley work? →
LUT: A lookup table: a list (1D) or cube (3D) of input colours and the output colours they become.See it in No. 066: How does colour grading work? →
Lymph node: A bean-shaped filter where immune cells meet germs carried in the lymph.See it in No. 049: How does the immune system work? →

M

Mach number: Speed divided by the speed of sound; above about 0.3, air's squashing matters.See it in No. L02: What is Bernoulli's principle? →
Magnetic domains: Tiny regions in iron where the atoms' magnets all point one way. Lining them up magnetises the iron.See it in No. L26: What is magnetism? →
Magnetic field: The region round a magnet or current where magnetic forces act, with a strength and a direction.See it in No. L26: What is magnetism? →
Magnetic flux: How much magnetic field passes through an area, in webers (Wb); for a flat coil Φ = B × A × cos θ.See it in No. L07: What is Faraday's law of induction? →
Magnetic poles: The north and south ends of a magnet. Like poles repel, unlike poles attract, and they always come in pairs.See it in No. L26: What is magnetism? →
Magnetosphere: The bubble of the Earth's field in space that turns aside the solar wind and guides the aurora.See it in No. L26: What is magnetism? →
Magnetron: A vacuum tube where electrons, bent by magnets, swirl past resonant cavities and make microwaves.See it in No. 008: How does a microwave oven work? →
Maillard reaction: Reactions between sugars and amino acids above about 140 °C that brown food and create flavour.See it in No. 017: How does an air fryer work? →
mAs: Tube current times exposure time. It sets how many photons are made.See it in No. 018: How does an X-ray machine work? →
Mass: The amount of inertia an object has, measured in kilograms: m = F ÷ a.See it in No. L17: What is inertia? →
Matchmove: Recreating the real camera's movement in the computer so CG moves with the plate.See it in No. 062: How do visual effects (VFX) work? →
Matte: A black-and-white picture that says how much of each pixel to keep.See it in No. 062: How do visual effects (VFX) work? →
Max pooling: Keeping the biggest number in each small block, which shrinks a map and tolerates small shifts.See it in No. 078: How does image recognition work? →
Mechanical advantage: How many times a machine multiplies your force. It always costs the same factor in distance.See it in No. L19: What are work and power? →
Mechanical advantage: How many times a machine multiplies force; for a hydraulic press, A₂ ÷ A₁.See it in No. L08: What is Pascal's principle? →
Melanin: The pigment made by melanocytes that colours skin and absorbs ultraviolet light.See it in No. 040: How does the skin work? →
Memory bandwidth: How many bytes per second can move from memory to the chip; it sets LLM decoding speed.See it in No. 077: How does AI inference work? →
Memory cell: A long-lived lymphocyte that answers fast if the same germ returns.See it in No. 049: How does the immune system work? →
Menstrual cycle: The monthly cycle of egg release and lining growth and shedding, usually 21 to 35 days.See it in No. 050: How does the reproductive system work? →
Mesentery: The fan-shaped fold of tissue that holds the intestines and carries their blood vessels.See it in No. 036: How do the intestines work? →
Mesh: The skin of a 3D model: vertices joined by edges into flat faces.See it in No. 060: How does 3D animation work? →
Micelle: A tiny ball of surfactant molecules, tails in and heads out, that can carry oil away in water.See it in No. 012: How does a washing machine work? →
MIDI: The 1983 standard that lets instruments and computers send notes to each other as short messages.See it in No. 027: How does a synthesizer work? →
Mileage: How far a vehicle goes on one litre of fuel, in km/L.See it in No. 021: How does a motorcycle work? →
Millisievert: A unit of radiation dose. The world average from natural sources is about 2.4 mSv a year.See it in No. 019: How does a CT scanner work? →
Millisievert (mSv): The unit of effective dose, weighting radiation by how much harm it can do to each organ.See it in No. 018: How does an X-ray machine work? →
Mizaj: Temperament: a person’s blend of hot or cold and moist or dry, as Unani describes it.See it in No. 082: How does Unani medicine work? →
Mode: One pattern a drumhead can vibrate in, with its own frequency and still nodal lines.See it in No. 025: How does a tabla work? →
Model sheet: A reference page with views, proportions, sizes and colour codes for the builders.See it in No. 053: How does concept art work? →
Modem: A modulator-demodulator that turns bits into sound and back.See it in No. 069: How does a fax machine work? →
Modified Huffman: The fax code book that gives short codes to common run lengths.See it in No. 069: How does a fax machine work? →
Moiré: False stripes when the LED grid and the camera sensor's grid interfere.See it in No. 064: How does virtual production work? →
Mole: A count of molecules: 6.022 × 10²³. A litre of air holds about 0.04 mol.See it in No. L06: What are the gas laws? →
Moment of inertia: Resistance to changes in spin, I = Σ m r², in kg·m². Mass far from the axis counts most.See it in No. L17: What is inertia? →
Momentum: Mass times velocity, p = m v, in kg·m/s. It has a size and a direction.See it in No. L18: What is momentum? →
Momentum: Mass × velocity. Forces change it, and in any push between two things it is shared, never lost.See it in No. L03: What are Newton's laws of motion? →
Mother tincture (Ø): The starting liquid for a plant remedy: plant material soaked in alcohol and filtered.See it in No. 081: How does homeopathy work? →
Motion capture: Recording a real performer's movement so a digital character can move the same way.See it in No. 063: How does motion capture work? →
Motivated light: Light that seems to come from a real source in the scene, like a window or lamp.See it in No. 055: How does film lighting work? →
Motor effect: A current in a field feels a force F = B I L at right angles to both, shown by Fleming's left-hand rule.See it in No. L26: What is magnetism? →
Mucociliary escalator: Sticky mucus plus beating cilia that carry trapped dirt up and out of the airways.See it in No. 042: How does the respiratory system work? →
Mucus barrier: A gel laced with bicarbonate that protects the stomach lining from its own acid.See it in No. 035: How does the stomach digest food? →
Mukkutram: The three humours, vatham, pitham and kabam, as Siddha describes them.See it in No. 084: How does Siddha medicine work? →
Multiplane camera: Artwork on levels at different distances from the camera, for parallax and depth.See it in No. 058: How does 2D animation work? →
Multiply-add (MAC): Multiply one input by one weight and add it to a running total; two FLOPs.See it in No. 077: How does AI inference work? →
Myelin: A fatty wrapping that makes the signal jump from node to node, much faster.See it in No. 045: How does the nervous system work? →
Myelin: Fatty insulation around many axons that makes spikes jump between gaps, the nodes of Ranvier, much faster.See it in No. 030: How does the brain work? →
Myopia: Short sight: distant things blur because light focuses in front of the retina, usually because the eye is too long.See it in No. 031: How does the eye see? →

N

Nasal cycle: The natural swapping of swelling between the two sides of the nose every few hours.See it in No. 033: How does the nose smell? →
Natural frequency: The rate an object vibrates at by itself, set by its stiffness and its mass.See it in No. 026: How does a harmonium work? →
Naturopathy: A drug-free system of healing that uses diet, fasting, water, mud, sun, air, massage and exercise.See it in No. 085: How does naturopathy work? →
ND filter: Neutral density glass that cuts light without changing colour. Each 0.3 of density is one stop.See it in No. 054: How does a cinema camera work? →
Negative feedback: When a hormone's rise turns down its own supply, holding the level steady.See it in No. 048: How does the endocrine system work? →
Nei kuri: The oil-drop test: how a drop of sesame oil spreads on a urine sample.See it in No. 084: How does Siddha medicine work? →
Nephron: The kidney's working unit: a glomerulus and a long tubule. About a million per kidney.See it in No. 039: How do the kidneys work? →
Net force: All the forces on an object added together as arrows. Only the net force changes motion.See it in No. L16: What is a force? →
Neuron: A nerve cell: dendrites collect signals and a long axon sends them on.See it in No. 045: How does the nervous system work? →
Neuron: A nerve cell that receives signals on its dendrites and sends spikes along its axon. The brain has about 86 billion.See it in No. 030: How does the brain work? →
Neuroplasticity: The brain's ability to change its connections with learning, practice and recovery.See it in No. 030: How does the brain work? →
Neurotransmitter: A chemical messenger such as glutamate (go), GABA (stop), dopamine or serotonin.See it in No. 030: How does the brain work? →
Neutral buoyancy: Exactly as dense as the surrounding fluid, so it neither rises nor sinks.See it in No. L23: What are density and buoyancy? →
Newton (N): The SI unit of force: it gives 1 kg an acceleration of 1 m/s². About the weight of a 100 g apple.See it in No. L16: What is a force? →
Newton-metre (N·m): The unit of torque: 1 newton pushing at right angles 1 metre from the pivot.See it in No. L24: What is torque? →
Nib: The metal point of a fountain pen, split into two tines by a thin slit.See it in No. 002: How does a pen work? →
Nit: A unit of screen brightness: one candela per square metre.See it in No. 066: How does colour grading work? →
Node: A point on a vibrating string that stays still.See it in No. 023: How does a guitar work? →
Non-ionising radiation: Radiation whose photons are too weak to knock electrons off atoms, like radio and visible light.See it in No. 071: How does a mobile phone work? →
Non-max suppression: Keeping the most confident box and deleting the boxes that overlap it too much.See it in No. 078: How does image recognition work? →
Normal: The line at right angles to a surface. Angles in Snell's law are measured from it.See it in No. L12: What is Snell's law of refraction? →
Normal force: How hard two surfaces are pressed together, at right angles to them. On a flat table it equals the weight.See it in No. L20: What is friction? →
Normal force: The push of a surface on something resting on it, at right angles to the surface.See it in No. L16: What is a force? →
NTC thermistor: A sensor whose electrical resistance falls as it gets hotter, used to measure temperature.See it in No. 017: How does an air fryer work? →

O

Occlusion: When a body part or object hides a marker from a camera.See it in No. 063: How does motion capture work? →
Oesophagus: The 25 cm muscular food pipe from the throat to the stomach.See it in No. 035: How does the stomach digest food? →
Ohmic: Obeying Ohm's law: current in proportion to voltage, a straight I–V line.See it in No. L01: What is Ohm's law? →
OLED: Organic light-emitting diode: a display where every subpixel is its own tiny light.See it in No. 014: How does a TV work? →
Olfactory bulb: The first stop for smell in the brain, where axons of one receptor type meet in the same glomeruli.See it in No. 033: How does the nose smell? →
Olfactory epithelium: The small patch of lining high in the roof of the nose that holds the smell neurons.See it in No. 033: How does the nose smell? →
Olfactory receptor: A protein on a smell neuron's cilia that binds certain molecules. Humans have about 400 working types.See it in No. 033: How does the nose smell? →
On twos: Holding each pose for two frames, so there are 12 new poses a second at 24 fps.See it in No. 059: How does stop-motion animation work? →
On twos: Each drawing held for two frames: 12 drawings a second.See it in No. 058: How does 2D animation work? →
On-device AI: Running a model on your own phone or laptop instead of in a data centre.See it in No. 077: How does AI inference work? →
Onion skin: Faded copies of earlier frames shown over the camera's live view.See it in No. 059: How does stop-motion animation work? →
Onion skin: Faint ghosts of the drawings before and after the one you are drawing.See it in No. 058: How does 2D animation work? →
Open–open pipe: A tube open at both ends. Its lowest note is f = v/2L, and it has every harmonic: 2f, 3f, 4f.See it in No. 024: How does a flute work? →
Operating system: The master program that runs the hardware and shares it between apps.See it in No. 074: How does a computer work? →
Oral rehydration salts: Salts and glucose in clean water, absorbed together even in bad diarrhoea.See it in No. 043: How does the digestive system work? →
Orbit: A path where an object keeps falling but moves sideways fast enough to keep missing the ground.See it in No. L21: What is gravity? →
ORS: Oral rehydration salts: salt and sugar that carry water back into the body during diarrhoea.See it in No. 044: How does the excretory system work? →
ORS: Oral rehydration solution: water with a measured amount of salts and sugar that the gut can absorb even during diarrhoea.See it in No. 036: How do the intestines work? →
Oscillator: A circuit or program that makes a repeating wave. Its frequency sets the note.See it in No. 027: How does a synthesizer work? →
Osmosis: The net flow of water through a semipermeable membrane towards the side with more dissolved particles.See it in No. L14: What are osmosis and diffusion? →
Osmotic pressure: The pressure needed to stop water flowing into a salty solution through a membrane. For salt water, π = iMRT.See it in No. 010: How does an RO water purifier work? →
Osmotic pressure (π): The pressure needed to stop water flowing in: π = iMRT. About 27 bar for seawater, 7.5 bar for blood.See it in No. L14: What are osmosis and diffusion? →
Ossicles: The malleus, incus and stapes: three tiny middle-ear bones that pass vibrations to the inner ear. The stapes is the body's smallest bone.See it in No. 032: How does the ear hear? →
Osteon: A tiny tube of bone rings round a blood vessel, about 0.2 mm across: the unit of compact bone.See it in No. 046: How does the skeletal system work? →
Outer frustum: The rest of the wall, drawn from a fixed point and used mainly for light and reflections.See it in No. 064: How does virtual production work? →
Ovary: An almond-sized organ that stores eggs and makes oestrogen and progesterone.See it in No. 050: How does the reproductive system work? →
Overblowing: Blowing harder with the same fingering so the tube jumps to a higher mode, often an octave up.See it in No. 024: How does a flute work? →
Overcranking: Shooting faster than playback, which gives slow motion. 120 fps played at 24 is 5× slower.See it in No. 054: How does a cinema camera work? →
Overfitting: Memorising the training examples, noise and all, instead of the general pattern.See it in No. 075: How do neural networks work? →
Overload protector: A heat-sensitive bimetal switch that cuts the motor's power when its windings get too hot.See it in No. 016: How does a mixer grinder work? →
Overtone: A higher vibration of a string in 2, 3, 4 or more parts, sounding along with the main note.See it in No. 006: How does a piano work? →
Ovulation: The release of an egg, about 14 days before the next period.See it in No. 050: How does the reproductive system work? →

P

Pacinian corpuscle: An onion-like touch receptor deep in the skin, most sensitive to vibration near 250 Hz.See it in No. 040: How does the skin work? →
Pager: A small one-way radio receiver that shows short messages sent to its own address.See it in No. 072: How does a pager work? →
Paint-over: Painting on top of a rendered 3D view to add colour, light and detail.See it in No. 053: How does concept art work? →
Pallet: The padded valve under each key that lets air through that key's reeds.See it in No. 026: How does a harmonium work? →
Pancha mahabhuta: The five great elements, space, air, fire, water and earth, that naturopathy says make up the body.See it in No. 085: How does naturopathy work? →
Papillae: The small bumps on the tongue; filiform papillae sense touch, the other three kinds carry taste buds.See it in No. 034: How does the tongue taste? →
Parallax: Near things shift more than far things when the viewpoint moves; it is how depth shows.See it in No. 064: How does virtual production work? →
Parameter: One learned number inside the model; large models have billions.See it in No. 076: How do large language models (LLMs) work? →
Parietal cell: A stomach cell that makes hydrochloric acid using proton pumps (H⁺/K⁺-ATPase).See it in No. 035: How does the stomach digest food? →
Parpam and chenduram: White and red powders made from minerals or metals by purification and firing.See it in No. 084: How does Siddha medicine work? →
Partial pressure: The share of a gas mixture's pressure due to one gas. Gases diffuse from high partial pressure to low.See it in No. 029: How do the lungs work? →
Partial pressure: Each gas in a mixture pushes as if it were alone, and the pushes add up (Dalton's law).See it in No. L06: What are the gas laws? →
Pascal (Pa): The SI unit of pressure: one newton per square metre. 1 bar = 100,000 Pa; 1 atm = 101,325 Pa.See it in No. L22: What is pressure? →
Pascal (Pa): The SI unit of pressure, one newton per square metre; 100,000 Pa is 1 bar.See it in No. L08: What is Pascal's principle? →
Pascal's principle: A change of pressure anywhere in a confined fluid is passed on undiminished to every part of it.See it in No. L08: What is Pascal's principle? →
Path loss: How much weaker a radio signal gets between tower and phone, in decibels.See it in No. 071: How does a mobile phone work? →
Path tracing: Averaging many random light paths per pixel to capture bounced light.See it in No. 061: How does CGI rendering work? →
Penetration depth: How far microwaves reach into a material before most of their power is used up: about 1.7 cm in water.See it in No. 008: How does a microwave oven work? →
Pepsin: An enzyme, activated by acid, that breaks proteins into smaller pieces.See it in No. 035: How does the stomach digest food? →
Performance capture: Capturing body, face and voice together so the whole acting performance is kept.See it in No. 063: How does motion capture work? →
Period: The time for one bounce of a mass on a spring: T = 2π √(m ÷ k).See it in No. L09: What is Hooke's law? →
Peripheral nervous system: All the nerves outside the brain and cord that link them to the body.See it in No. 045: How does the nervous system work? →
Peristalsis: A wave of muscle squeezing that pushes food along the gut.See it in No. 043: How does the digestive system work? →
Peristalsis: A travelling wave of muscle contraction that pushes food along.See it in No. 036: How do the intestines work? →
Peristalsis: Waves of muscle squeezing that push food along the gut.See it in No. 035: How does the stomach digest food? →
Perpetual motion: A machine that runs for ever or makes energy from nothing. Conservation of energy rules it out.See it in No. L04: What is conservation of energy? →
Phagocytosis: When a neutrophil or macrophage swallows and digests a germ.See it in No. 049: How does the immune system work? →
Photoconductor: A coating that holds static charge in the dark but lets it leak away where light hits it.See it in No. 068: How does a printer work? →
Pickup: A magnet inside a coil of fine wire that turns a steel string's motion into a small voltage.See it in No. 023: How does a guitar work? →
Picture lock: The point where the edit is final and the film moves on to VFX, grading and the sound mix.See it in No. 065: How does film editing work? →
Piezo inkjet: A printhead that squeezes out drops with a crystal that bends when a voltage is applied.See it in No. 068: How does a printer work? →
Pitch: Table travel per rotation divided by the beam width. At pitch 1 the spiral's turns just touch.See it in No. 019: How does a CT scanner work? →
Pitch: How steeply a blade is tilted from the plane it spins in. Ceiling fans use about 10 to 14 degrees.See it in No. 004: How does a fan work? →
Pituitary: The pea-sized master gland under the brain whose hormones switch other glands on.See it in No. 048: How does the endocrine system work? →
Pixel: One tiny square of a picture, stored as red, green and blue numbers.See it in No. 078: How does image recognition work? →
Pixel: One dot of the picture, made of a red, a green and a blue subpixel.See it in No. 014: How does a TV work? →
Pixel pitch: The distance between neighbouring LED pixels, around 2.3 to 2.9 mm on film stages.See it in No. 064: How does virtual production work? →
Pixilation: Stop-motion animation of real people, who pose for every frame.See it in No. 059: How does stop-motion animation work? →
Placebo: A dummy treatment that looks identical but has no active ingredient.See it in No. 083: How does modern medicine (allopathy) work? →
Placebo: A look-alike dummy treatment; comparing against it shows what a remedy really adds.See it in No. 082: How does Unani medicine work? →
Placebo: A dummy treatment. Fair tests compare a remedy with one to see if it adds anything.See it in No. 081: How does homeopathy work? →
Placebo: A dummy treatment used in fair trials to show what the real treatment adds.See it in No. 080: How does Ayurveda work? →
Placebo effect: Feeling better because you expect to, which is why fair tests compare against a control group.See it in No. 085: How does naturopathy work? →
Placenta: The organ on the uterus wall where the baby's blood takes oxygen and food from the mother's.See it in No. 050: How does the reproductive system work? →
Plastic deformation: A permanent change of shape once the elastic limit is passed.See it in No. L09: What is Hooke's law? →
Plate: The original live-action footage a VFX shot is built on.See it in No. 062: How do visual effects (VFX) work? →
PM2.5: Airborne particles smaller than 2.5 micrometres, small enough to reach the alveoli. The WHO yearly guideline is 5 micrograms per cubic metre.See it in No. 029: How do the lungs work? →
POCSAG: The pager code standardised in 1981: 32-bit codewords, sent in batches of a sync word and 8 frames.See it in No. 072: How does a pager work? →
Poiseuille's law: Flow through a narrow tube depends on its radius to the fourth power.See it in No. 041: How does the circulatory system work? →
Polar pattern: How sensitive a microphone is in each direction; a shotgun hears mostly straight ahead.See it in No. 057: How do film sound and Foley work? →
Polarizer: A film that only lets through light waves shaking in one direction.See it in No. 014: How does a TV work? →
Portal vein: The vein that takes blood from the gut to the liver before anywhere else.See it in No. 043: How does the digestive system work? →
Portal vein: The vein that brings blood from the stomach, intestines and spleen to the liver, about three-quarters of its supply.See it in No. 037: How does the liver work? →
Positive predictive value: Of all positive test results, the share who really have the disease.See it in No. 083: How does modern medicine (allopathy) work? →
Potential energy: Energy stored by position or shape, like height (m × g × h) or a stretched string.See it in No. L04: What is conservation of energy? →
Potentisation: Repeated dilution with vigorous shaking (succussion). 30C means 1 part in 10⁶⁰.See it in No. 081: How does homeopathy work? →
Power: How fast work is done: work ÷ time, or force × speed.See it in No. L19: What are work and power? →
Power: Energy per second, in watts: P = V × I = I² × R.See it in No. L01: What is Ohm's law? →
Power = τω: For anything that turns, power is torque × angular speed in radians per second.See it in No. L24: What is torque? →
Power window: A soft shape that limits where a correction applies, often tracked to a moving subject.See it in No. 066: How does colour grading work? →
Prakriti: A person's inborn constitution: their own mix of the three doshas.See it in No. 080: How does Ayurveda work? →
Pre-vis: Previsualisation: planning shots with a simple 3D model of the set, actors and cameras before shooting.See it in No. 052: How do screenplays and storyboards work? →
Precession: The slow swing of a spinning object's axis when a torque acts at right angles to its spin.See it in No. L24: What is torque? →
Precession: The wobble of a spinning proton round the direction of the magnetic field.See it in No. 020: How does an MRI machine work? →
Prefill and decode: Processing the whole prompt at once, then writing the answer one token at a time.See it in No. 077: How does AI inference work? →
Presbyopia: The loss of near focusing with age as the lens stiffens, usually noticed in the 40s.See it in No. 031: How does the eye see? →
Pressure: Force divided by the area it acts on, P = F ÷ A.See it in No. L22: What is pressure? →
Pressure: Force on each unit of area, P = F ÷ A, measured in pascals (N/m²).See it in No. L08: What is Pascal's principle? →
Pressure: The push of a gas on each square metre of wall, from its molecules hitting it. Air around you is about 101 kPa.See it in No. L06: What are the gas laws? →
Pressure-relief valve: A spring-loaded valve that lets water out when heating makes it expand and the pressure climbs too high.See it in No. 022: How does a water heater (geyser) work? →
Primary air: Air mixed with the gas inside the burner before it reaches the flame.See it in No. 011: How do a kitchen chimney and gas hob work? →
Principle of moments: For balance, the clockwise moments about a pivot equal the anticlockwise moments.See it in No. L24: What is torque? →
Printhead: The chip in an inkjet with hundreds of tiny nozzles that fire drops of ink.See it in No. 068: How does a printer work? →
Production sound: The dialogue and sound recorded on set while filming.See it in No. 057: How do film sound and Foley work? →
Program counter: The register that holds the address of the next instruction.See it in No. 074: How does a computer work? →
Projection: The shadow profile of a slice from one angle: how much X-ray each detector received.See it in No. 019: How does a CT scanner work? →
Prosthetic appliance: A soft foam latex or silicone piece glued on to change an actor's face.See it in No. 056: How do practical special effects work? →
Proving: A test in which healthy volunteers take a substance and record their symptoms.See it in No. 081: How does homeopathy work? →
Puberty: The years when the body matures, started by GnRH from the brain.See it in No. 050: How does the reproductive system work? →
Publication bias: When dull or negative trials go unpublished, making treatments look better than they are.See it in No. 083: How does modern medicine (allopathy) work? →
Pudam: Firing sealed clay saucers in a pit of burning cow-dung cakes.See it in No. 084: How does Siddha medicine work? →
Pulse dialling: Sending a digit by breaking the line current that many times, about 10 times a second.See it in No. 070: How does a telephone work? →
Pulse-width modulation: Making in-between brightness by switching fully on and off and varying the on time.See it in No. 067: How does a cinema projector work? →
PWM: Pulse-width modulation: fast on-off pulses whose changing widths average out to a smooth wave.See it in No. 009: How does an inverter or UPS work? →
Pylorus: The stomach's exit, whose sphincter lets through only small particles.See it in No. 035: How does the stomach digest food? →

Q

QAM: A way of sending several bits per symbol by setting a wave's size and phase to one of many grid points.See it in No. 071: How does a mobile phone work? →
Qualifier: A selection made from a range of hue, saturation and brightness.See it in No. 066: How does colour grading work? →
Quantisation: Storing weights with fewer bits by rounding them onto a few allowed levels.See it in No. 077: How does AI inference work? →
Quantum dot: A crystal a few nanometres across that turns blue light into a pure colour set by its size.See it in No. 014: How does a TV work? →

R

Radiation: Heat carried by light, mostly infrared. It crosses empty space and grows as the fourth power of temperature in kelvin.See it in No. L10: How does heat move? →
Randomised blinded trial: A test where chance decides who gets what and nobody knows, so bias can't creep in.See it in No. 080: How does Ayurveda work? →
Randomised controlled trial: A test where chance decides who gets the treatment and who gets the comparison.See it in No. 083: How does modern medicine (allopathy) work? →
Rasa shastra: Ayurvedic pharmacy that processes metals and minerals into medicines.See it in No. 080: How does Ayurveda work? →
Rasterising: Drawing by projecting triangles onto the screen and filling their pixels, the way games do.See it in No. 061: How does CGI rendering work? →
Ray: A straight line fired from the camera through a pixel into the scene, or from a surface towards a light.See it in No. 061: How does CGI rendering work? →
RCCB: A breaker that cuts the power when 30 mA or more leaks to earth, for example through a person.See it in No. L27: What are electric current and voltage? →
Reabsorption: Taking water and useful substances back from the tubule into the blood; about 99% of the water.See it in No. 039: How do the kidneys work? →
Reaction force: The push back on a nozzle as its jet shoots water out. It is what turns the spray arms.See it in No. 013: How does a dishwasher work? →
Real contact area: The tiny fraction of a surface that actually touches, at the tips of its bumps. It grows with the load.See it in No. L20: What is friction? →
Receptor: A protein on or in a cell that one hormone fits, like a lock and its key.See it in No. 048: How does the endocrine system work? →
Receptor: A protein on a cell that catches one kind of chemical and switches the cell on.See it in No. 034: How does the tongue taste? →
Recirculation: Pumping the same few litres from the sump to the spray arms over and over during one stage.See it in No. 013: How does a dishwasher work? →
Recovery: The share of the incoming water that comes out purified. The rest leaves as reject.See it in No. 010: How does an RO water purifier work? →
Rectifier: A circuit, usually four diodes in a bridge, that turns AC into current flowing one way.See it in No. 009: How does an inverter or UPS work? →
Red marrow: Soft tissue inside bones that makes red cells, white cells and platelets.See it in No. 046: How does the skeletal system work? →
Reflex arc: Sensor, sensory nerve, spinal cord, motor nerve, muscle: an automatic loop that skips the brain.See it in No. 045: How does the nervous system work? →
Refraction: The bending of light as it crosses from one material into another, because its speed changes.See it in No. L12: What is Snell's law of refraction? →
Refractive index (n): How many times slower light goes in a material than in a vacuum: air 1.000, water 1.333, glass about 1.5, diamond 2.42.See it in No. L12: What is Snell's law of refraction? →
Refrigerant: The fluid that circulates in a fridge, boiling inside to absorb heat and condensing outside to release it. Most new fridges use isobutane, R600a.See it in No. 015: How does a refrigerator work? →
Refrigerant: The fluid that circulates in an AC, boiling indoors to absorb heat and condensing outdoors to release it. Many new ACs use R-32.See it in No. 005: How does an air conditioner work? →
Regenerative braking: Slowing an electric car by using its motor as a generator, putting energy back into the battery instead of wasting it as heat.See it in No. 007: How does a car work? →
Regression to the mean: After an unusually bad day, the next ones tend to be closer to normal, with or without treatment.See it in No. 081: How does homeopathy work? →
Regulator: The valve on the cylinder that lowers the gas pressure to a steady 29 millibar for the stove.See it in No. 011: How do a kitchen chimney and gas hob work? →
Remodelling: Osteoclasts removing old bone and osteoblasts laying down new bone, all life long.See it in No. 046: How does the skeletal system work? →
Render farm: Many computers rendering different frames of a film at the same time.See it in No. 061: How does CGI rendering work? →
Render farm: Hundreds or thousands of computers rendering frames at the same time.See it in No. 060: How does 3D animation work? →
Renin: A kidney enzyme released when blood pressure falls; it starts the chain to angiotensin II and aldosterone.See it in No. 039: How do the kidneys work? →
Repertory: An index of symptoms, each listing the remedies linked to it.See it in No. 081: How does homeopathy work? →
Replacement animation: Swapping pre-made pieces, such as mouths or faces, between frames.See it in No. 059: How does stop-motion animation work? →
Reprojection error: How far, in pixels, solved 3D points land from the real tracks. Under half a pixel is good.See it in No. 062: How do visual effects (VFX) work? →
Reserpine: A blood-pressure drug isolated in 1952 from the Ayurvedic plant sarpagandha.See it in No. 080: How does Ayurveda work? →
Reservoir: The inner, spring-loaded bellows that stores air and keeps the wind pressure steady.See it in No. 026: How does a harmonium work? →
Resistance: How strongly something opposes current, measured in ohms (Ω). One volt drives one amp through one ohm.See it in No. L01: What is Ohm's law? →
Resistivity: A material's own resistance for a 1 m cube; wire resistance is resistivity × length ÷ area.See it in No. L01: What is Ohm's law? →
Resolution: Dots per inch across and down the page: 204 × 98 standard, 204 × 196 fine.See it in No. 069: How does a fax machine work? →
Resonance: A boost at a filter's cutoff, made by feeding its output back in.See it in No. 027: How does a synthesizer work? →
Resonance: The speed at which a tub on springs naturally shakes the most, around 200 rpm in a front-loader.See it in No. 012: How does a washing machine work? →
Resonance: A large response when a system is driven at its own natural frequency.See it in No. L11: What are waves and resonance? →
Respiratory system: The organs that move air in and out and swap oxygen for carbon dioxide: nose, throat, larynx, trachea, bronchi, lungs and the breathing muscles.See it in No. 042: How does the respiratory system work? →
Retargeting: Copying motion from one skeleton onto a body with different proportions.See it in No. 063: How does motion capture work? →
Retina: The light-sensing layer lining the back of the eye, with rods, cones and nerve cells.See it in No. 031: How does the eye see? →
Retronasal smell: Smelling food from inside the mouth as air flows up into the nose; most of flavour.See it in No. 034: How does the tongue taste? →
Retronasal smell: Smelling food from inside the mouth, as air rises up the back of the nose while you chew and breathe out.See it in No. 033: How does the nose smell? →
Retroreflective marker: A small ball with tape that bounces light straight back to the camera that lit it.See it in No. 063: How does motion capture work? →
Reverse osmosis: Pushing on salty water harder than its osmotic pressure, so pure water is squeezed out through a membrane.See it in No. L14: What are osmosis and diffusion? →
Reverse osmosis: Pushing water through a semi-permeable membrane harder than the osmotic pressure, so fresh water leaves the dissolved salts behind.See it in No. 010: How does an RO water purifier work? →
RF pulse: A short burst of radio waves at the Larmor frequency that tips the spins over.See it in No. 020: How does an MRI machine work? →
Ringing current: An alternating voltage of about 75 V at 25 Hz that the exchange sends to ring a phone.See it in No. 070: How does a telephone work? →
Rocket equation: Δv = v_e ln(m_full ÷ m_empty), Tsiolkovsky, 1903.See it in No. L18: What is momentum? →
Rods and cones: Rods see in dim light without colour; three kinds of cone see colour and detail in good light.See it in No. 031: How does the eye see? →
Rolling resistance: The small drag on a rolling wheel, Crr × weight: about 1% for a car tyre.See it in No. L20: What is friction? →
Rotary shutter: A spinning bladed disc that blocks light while the film moves and flashes each frame, usually twice.See it in No. 067: How does a cinema projector work? →
Rotoscoping: Drawing shapes round a moving object on keyframes to make a matte.See it in No. 062: How do visual effects (VFX) work? →
Run-length encoding: Describing a line by how many white and black dots come in a row.See it in No. 069: How does a fax machine work? →

S

Sa: The home note of Indian music. A bansuri is named by its Sa, played with the top three holes closed.See it in No. 024: How does a flute work? →
SA node: The heart's natural pacemaker, which fires each beat 60 to 100 times a minute at rest.See it in No. 028: How does the heart work? →
Sacrificial anode: A magnesium rod that corrodes on purpose, protecting the steel tank it is bolted into.See it in No. 022: How does a water heater (geyser) work? →
Salt rejection: The share of dissolved salt a membrane holds back, usually 95 to 99%.See it in No. 010: How does an RO water purifier work? →
Samples per pixel: How many random paths are averaged for each pixel. Noise falls as 1/√N.See it in No. 061: How does CGI rendering work? →
SAR: Specific absorption rate: radio power absorbed by the body, limited in India to 1.6 W/kg over 1 g of tissue.See it in No. 071: How does a mobile phone work? →
Sarcomere: The repeating unit of a muscle fibre, about 2–2.5 µm long, from one Z-disc to the next.See it in No. 047: How do your muscles move you? →
Sarcopenia: Loss of muscle mass and strength with age.See it in No. 047: How do your muscles move you? →
Scale: Calcium carbonate that comes out of hard water when it is heated and coats the element.See it in No. 022: How does a water heater (geyser) work? →
Scan line: One thin strip across the page: 1,728 dots in a Group 3 fax.See it in No. 069: How does a fax machine work? →
Scene heading: The line that opens a scene: INT. or EXT., the place and the time of day, such as EXT. ROOFTOP - DAY.See it in No. 052: How do screenplays and storyboards work? →
Scintillator: A material, like caesium iodide, that turns an X-ray into a flash of visible light.See it in No. 018: How does an X-ray machine work? →
Scoville heat unit: A measure of chilli heat; pure capsaicin is about 16 million SHU.See it in No. 034: How does the tongue taste? →
Screen gain: How much brighter a screen looks straight on than matte white; silver is about 2.4.See it in No. 067: How does a cinema projector work? →
Screenplay: A film script in a standard layout: 12-point Courier, fixed margins, roughly one page per minute of screen time.See it in No. 052: How do screenplays and storyboards work? →
Secretin and CCK: Hormones from the duodenum that tell the pancreas to send bicarbonate and enzymes when food arrives.See it in No. 038: How does the pancreas work? →
Self-stability: A moving bicycle's ability to correct a lean without a rider.See it in No. 003: How does a bicycle work? →
Semi-permeable membrane: A skin that lets water molecules through but blocks dissolved ions.See it in No. 010: How does an RO water purifier work? →
Semicircular canals: Three fluid-filled loops in the inner ear that sense the head turning.See it in No. 032: How does the ear hear? →
Semipermeable membrane: A sheet that lets water through but blocks dissolved salt or sugar.See it in No. L14: What are osmosis and diffusion? →
Sensor noise: Random speckle from the physics of counting light. ISO amplification makes it more visible.See it in No. 001: How does a camera actually see? →
Series and parallel: One path shared by every part, or a separate path for each part across the supply.See it in No. L27: What are electric current and voltage? →
Set extension: Adding painted or CG scenery beyond the part of a set that was really built.See it in No. 062: How do visual effects (VFX) work? →
Seven-segment display: Seven bars that together can draw any digit from 0 to 9.See it in No. 073: How does a calculator work? →
Shift drum: A drum with curving grooves that moves the selector forks one gear at a time.See it in No. 021: How does a motorcycle work? →
Shooting ratio: Hours of footage shot for each hour of finished film.See it in No. 051: How does a film get made? →
Short-chain fatty acids: Acids such as butyrate made by gut bacteria from fibre; they fuel the colon's lining.See it in No. 036: How do the intestines work? →
Shot size: How much of the subject fills the frame: extreme wide, wide, medium, close-up or extreme close-up.See it in No. 052: How do screenplays and storyboards work? →
Shruti: A fine step of pitch in Indian music, finer than a semitone; tradition counts 22 in an octave.See it in No. 026: How does a harmonium work? →
Shutter angle: The fraction of each frame the shutter is open, as a slice of a 360° disc. 180° at 24 fps is 1/48 s.See it in No. 054: How does a cinema camera work? →
Shutter angle: How much of each frame the shutter is open: 180° means half, 1/48 s at 24 fps.See it in No. 051: How does a film get made? →
Siddhar: A sage of the Tamil tradition, remembered as a master of yoga, alchemy and medicine.See it in No. 084: How does Siddha medicine work? →
Silhouette test: Checking that a design can still be recognised as a solid black shape.See it in No. 053: How does concept art work? →
SIM: A tiny smart card holding a secret key that proves to the network who you are.See it in No. 071: How does a mobile phone work? →
Similia similibus curentur: "Let like be cured by like": homeopathy's principle that what causes symptoms in the healthy treats them in the sick.See it in No. 081: How does homeopathy work? →
Simulcast: Several transmitters sending the same signal at the same moment to cover a whole area.See it in No. 072: How does a pager work? →
Sinogram: All the projections of a slice stacked by angle; each point in the body traces a sine-shaped line in it.See it in No. 019: How does a CT scanner work? →
Skeletal muscle: Striped, voluntary muscle joined to bones by tendons.See it in No. 047: How do your muscles move you? →
Skin weight: How strongly a vertex follows each bone; a vertex's weights add up to 100%.See it in No. 060: How does 3D animation work? →
Slip: How far an induction motor's rotor lags behind its rotating field.See it in No. L07: What is Faraday's law of induction? →
Slip: How far an induction motor's rotor lags behind its rotating magnetic field, usually a few percent.See it in No. 004: How does a fan work? →
Slow- and fast-twitch fibres: Type I fibres are steady and tireless; type II are quick and powerful but tire fast.See it in No. 047: How do your muscles move you? →
SMS: A 140-byte text message carried on the network's control channel: 160 letters, or 70 in Unicode.See it in No. 071: How does a mobile phone work? →
Solving: Fitting a skeleton inside the labelled markers to get joint rotations.See it in No. 063: How does motion capture work? →
Soundboard: The thin wooden top of an acoustic guitar that the bridge shakes to push the air.See it in No. 023: How does a guitar work? →
Soundboard: A thin, wide sheet of spruce under the strings that turns their vibration into sound.See it in No. 006: How does a piano work? →
Space-time patch: A small block of the latent cube, a few cells wide and a few frames long, used as one token.See it in No. 079: How do AI video models work? →
Spacing: How far something moves from one frame to the next.See it in No. 059: How does stop-motion animation work? →
Special effects (SFX): Physical effects created for real on set and captured in camera.See it in No. 056: How do practical special effects work? →
Specific gravity: The density of a lead-acid battery's acid compared with water: about 1.26 when full, 1.12 when flat.See it in No. 009: How does an inverter or UPS work? →
Specific heat: The energy needed to warm 1 kg of a substance by 1 °C. For water, about 4,180 joules.See it in No. 022: How does a water heater (geyser) work? →
Specific heat: The energy to warm 1 kg by 1 °C: 4.19 kJ for water, which is why hot washes cost so much.See it in No. 012: How does a washing machine work? →
Speed: How far something travels per second: distance ÷ time, in metres per second.See it in No. L15: What are speed, velocity and acceleration? →
SPF: Sun protection factor: with SPF 30 applied as in the lab, about 1/30 of sunburning UV gets through.See it in No. 040: How does the skin work? →
Sphincter: A ring of muscle that closes a tube. The bladder has an automatic one and one you control.See it in No. 044: How does the excretory system work? →
Spill: Coloured light bouncing off a green or blue screen onto the actor.See it in No. 062: How do visual effects (VFX) work? →
Spoke tension: The pull built into every spoke, which lets thin wires carry a rider's weight.See it in No. 003: How does a bicycle work? →
Spring constant: A spring's stiffness, k, in newtons per metre: the force needed for each metre of stretch.See it in No. L09: What is Hooke's law? →
Squash and stretch: Flattening on impact and stretching in fast motion, keeping the same volume.See it in No. 058: How does 2D animation work? →
Standing loss: Heat a full, hot tank loses in a day with no water drawn; the basis of India's BEE star rating for geysers.See it in No. 022: How does a water heater (geyser) work? →
Standing wave: A wave that seems to stand still in the tube, with nodes where the pressure never changes.See it in No. 024: How does a flute work? →
Standing wave: Two waves travelling opposite ways that add into a pattern of fixed nodes and antinodes.See it in No. L11: What are waves and resonance? →
Standing wave: A wave pattern that stays in place, made when waves reflect and overlap, with fixed hot spots and nodes.See it in No. 008: How does a microwave oven work? →
Starling forces: Blood pressure pushing fluid out of capillaries and plasma proteins pulling it back in.See it in No. 041: How does the circulatory system work? →
Static and kinetic friction: Static friction stops sliding from starting, up to μs × N; kinetic friction acts once sliding, about μk × N.See it in No. L20: What is friction? →
Static pressure: The push a fluid exerts on the walls around it, measured by a tube or gauge on the side.See it in No. L02: What is Bernoulli's principle? →
Stem: One group of sounds in a mix, such as all the dialogue or all the music.See it in No. 057: How do film sound and Foley work? →
Stick–slip: Jerky motion where surfaces stick, build up force, slip and stick again: violins, squeaks and earthquakes.See it in No. L20: What is friction? →
Stop: A knob that opens or closes the air to one bank of reeds, or to a drone reed.See it in No. 026: How does a harmonium work? →
Stop: A doubling or halving of light. Photographers measure every exposure change in stops.See it in No. 001: How does a camera actually see? →
Storyboard: A sequence of drawings, one per shot, showing what the camera will see.See it in No. 052: How do screenplays and storyboards work? →
Stratification: Water settling into layers by temperature, hottest on top, because warm water is less dense.See it in No. 022: How does a water heater (geyser) work? →
Stratum corneum: The outermost skin: flat dead cells of keratin glued by fats, the main barrier.See it in No. 040: How does the skin work? →
Streamline: The path a bit of fluid follows in a steady flow. Bernoulli holds along it.See it in No. L02: What is Bernoulli's principle? →
Stroke: Brain damage from a blocked or burst artery. Warning signs: BE FAST.See it in No. 030: How does the brain work? →
Stroke volume: The blood one ventricle pumps per beat, about 70 mL at rest.See it in No. 028: How does the heart work? →
Strowger switch: An electromechanical switch that steps up and round, one step per dial pulse.See it in No. 070: How does a telephone work? →
Subdivision surface: A smooth surface made by repeatedly splitting and smoothing a blocky cage of quads.See it in No. 060: How does 3D animation work? →
Subtractive synthesis: Making a sound by starting with a bright wave and filtering harmonics away.See it in No. 027: How does a synthesizer work? →
Sump: The small well under the tub floor where wash water collects before the pump sends it round again.See it in No. 013: How does a dishwasher work? →
Superconductor: A material that, when cold enough, carries current with zero resistance, so the magnet stays on without a power supply.See it in No. 020: How does an MRI machine work? →
Superconductor: A material whose resistance falls to exactly zero below a critical temperature.See it in No. L01: What is Ohm's law? →
Supertaster: A person who finds certain bitter chemicals, like PROP, very intense; about a quarter of people.See it in No. 034: How does the tongue taste? →
Surface tension: The pull between water molecules that makes drops round. Rinse aid weakens it so water sheets off.See it in No. 013: How does a dishwasher work? →
Surface tension: The pull between molecules at a liquid's surface that makes it behave like a stretched skin.See it in No. 002: How does a pen work? →
Surfactant: A soapy film lining the alveoli that lowers surface tension so they don't collapse.See it in No. 029: How do the lungs work? →
Surfactant: A molecule with a water-loving head and an oil-loving tail: the working part of soap and detergent.See it in No. 012: How does a washing machine work? →
Sustain pedal: The right pedal, which lifts every damper so notes keep ringing and other strings resonate.See it in No. 006: How does a piano work? →
Syahi: The black spot on the head: layers of paste with iron filings that load the centre of the skin.See it in No. 025: How does a tabla work? →
Sympathetic and parasympathetic: The fight-or-flight and rest-and-digest branches of the automatic nervous system.See it in No. 045: How does the nervous system work? →
Synapse: The junction where a neuron passes its message on with a chemical neurotransmitter.See it in No. 045: How does the nervous system work? →
Synapse: The junction where one neuron passes a signal to another, usually by releasing a neurotransmitter.See it in No. 030: How does the brain work? →
Synovial joint: A moving joint with cartilage-capped ends, a capsule and slippery fluid inside.See it in No. 046: How does the skeletal system work? →
System curve: How much pressure a duct needs for each airflow. Where it meets the fan curve is the real airflow.See it in No. 011: How do a kitchen chimney and gas hob work? →

T

T-stop: An aperture number corrected for light lost in the lens, so all cinema lenses expose alike.See it in No. 054: How does a cinema camera work? →
T.38: The standard for sending fax data, rather than fax sound, over the internet.See it in No. 069: How does a fax machine work? →
T1 and T2: How quickly magnetisation grows back along the field (T1) and how quickly the sideways signal fades (T2). Every tissue has its own.See it in No. 020: How does an MRI machine work? →
Tabiyat: In Unani, the body’s own power to keep itself in balance and heal.See it in No. 082: How does Unani medicine work? →
Taste bud: An onion-shaped cluster of about 50 to 150 cells that detect taste chemicals dissolved in saliva.See it in No. 034: How does the tongue taste? →
TDS: Total dissolved solids: everything dissolved in water, in mg per litre (ppm).See it in No. 010: How does an RO water purifier work? →
Teentaal: A 16-beat rhythmic cycle in four sections, with a khali (empty) section where the bayan rests.See it in No. 025: How does a tabla work? →
Temperature: A sampling dial: low picks the likeliest token, high spreads the choice out.See it in No. 076: How do large language models (LLMs) work? →
Temperature: How fast molecules jiggle on average. In kelvin it starts at absolute zero, −273.15 °C.See it in No. L05: What are the laws of thermodynamics? →
Temporal consistency: Objects staying the same from frame to frame unless they should change.See it in No. 079: How do AI video models work? →
Tendon: A tough cord of collagen that joins a muscle to a bone.See it in No. 047: How do your muscles move you? →
Tension: The pull carried along a rope, cable or chain.See it in No. L16: What is a force? →
Tesla: The unit of magnetic field strength. Earth's field is about 0.00005 T; hospital scanners are 1.5 or 3 T.See it in No. 020: How does an MRI machine work? →
Tesla (T): The SI unit of magnetic field (flux density). Earth ≈ 50 µT; an MRI 1.5–3 T.See it in No. L26: What is magnetism? →
Testis: An organ that makes sperm and testosterone, kept a little cooler than the body.See it in No. 050: How does the reproductive system work? →
Therapeutic window: The range of drug levels between too little to work and enough to harm.See it in No. 083: How does modern medicine (allopathy) work? →
Thermal conductivity: How easily a material lets heat through. PU foam is about 0.022 W/m·K; a vacuum panel about 0.004.See it in No. 015: How does a refrigerator work? →
Thermal conductivity (k): How well a material conducts heat, in W/m·K: copper 400, steel 16, wood 0.15, still air 0.026.See it in No. L10: How does heat move? →
Thermal effusivity: How strongly a material pulls heat from something touching it, √(kρc). It's why metal feels colder than wood.See it in No. L10: How does heat move? →
Thermal equilibrium: Things in touch at the same temperature, with no more heat flowing between them.See it in No. L05: What are the laws of thermodynamics? →
Thermal inkjet: A printhead that boils a tiny amount of ink so the vapour bubble pushes out a drop.See it in No. 068: How does a printer work? →
Thermal paper: Paper coated with a dye and developer that turn black when heated.See it in No. 069: How does a fax machine work? →
Thermal resistance (R): How hard a layer makes it for heat to pass: R = d ÷ k. Layers in a row add up.See it in No. L10: How does heat move? →
Thermocline: The thin boundary between hot water above and cold water below in a tank.See it in No. 022: How does a water heater (geyser) work? →
Thermocouple: Two different metals joined at a tip that make a small voltage when the tip is heated.See it in No. 011: How do a kitchen chimney and gas hob work? →
Thermostat: A switch worked by temperature that turns the heater off at the set point and back on when the water cools.See it in No. 022: How does a water heater (geyser) work? →
Thermostat: A switch that turns the heater on and off to hold a set temperature.See it in No. 017: How does an air fryer work? →
Thin-film composite: An RO membrane made of an ultra-thin polyamide layer on a porous support, rolled into a spiral-wound element.See it in No. 010: How does an RO water purifier work? →
Three-act structure: Set-up, confrontation and resolution, split roughly a quarter, a half and a quarter of the script.See it in No. 052: How do screenplays and storyboards work? →
Throw ratio: Distance to the screen divided by picture width, set by the lens focal length.See it in No. 067: How does a cinema projector work? →
Thrust: The push from throwing mass away: mass per second × speed. Rockets and fans both work this way.See it in No. L18: What is momentum? →
Thumbnail: A tiny, quick sketch used to try out many ideas fast.See it in No. 053: How does concept art work? →
Tidal volume: The air moved in one quiet breath, about 0.5 L for an adult.See it in No. 042: How does the respiratory system work? →
Tidal volume: The air moved in one quiet breath, about 500 mL for an adult at rest.See it in No. 029: How do the lungs work? →
Tides: Two ocean bulges raised by the difference in the Moon's (and Sun's) pull across the Earth.See it in No. L21: What is gravity? →
Tie-down: A screw or magnet that locks a puppet's foot to the set.See it in No. 059: How does stop-motion animation work? →
Time-division multiplexing: Many calls taking turns on one line, a few bits each, thousands of times a second.See it in No. 070: How does a telephone work? →
Timeline: The editor's workspace: shots laid end to end on picture and sound tracks.See it in No. 065: How does film editing work? →
Tip speed: How fast the end of a blade moves: v = ω × r, turning speed times radius.See it in No. 016: How does a mixer grinder work? →
Token: A numbered piece of text, such as a word, part of a word or a byte, that the model reads.See it in No. 076: How do large language models (LLMs) work? →
Toner: A fine powder of plastic and pigment that laser printers place with static and melt onto paper.See it in No. 068: How does a printer work? →
Tonicity: Whether a liquid makes cells swell (hypotonic), stay the same (isotonic) or shrink (hypertonic).See it in No. L14: What are osmosis and diffusion? →
Tonotopy: The map of pitch by place: high frequencies at the cochlea's base, low at its apex.See it in No. 032: How does the ear hear? →
Torque: The turning effect of a force: force × distance from the pivot, at right angles.See it in No. L24: What is torque? →
Torque: A twisting push. A force far from the centre twists more.See it in No. 013: How does a dishwasher work? →
Torque: Twisting force: force times the length of the lever arm, measured in newton metres (N·m) or pound-feet (lb·ft).See it in No. 007: How does a car work? →
Torque: A turning force: force multiplied by the length of the lever it pushes on.See it in No. 003: How does a bicycle work? →
Total internal reflection: All the light reflecting back at a boundary, past the critical angle. It traps light in optical fibres.See it in No. L12: What is Snell's law of refraction? →
Trail: How far the front tyre touches the road behind the point where the steering axis meets it. It keeps the steering steady.See it in No. 021: How does a motorcycle work? →
Trail: How far behind the steering axis the front tyre touches the ground. It helps the front wheel steer into a lean.See it in No. 003: How does a bicycle work? →
Transfer time: The gap between the mains failing and the backup taking over: about 10 ms for an offline UPS, zero for an online one.See it in No. 009: How does an inverter or UPS work? →
Transformer: The 2017 architecture that stacks attention and small neural networks; the basis of modern LLMs.See it in No. 076: How do large language models (LLMs) work? →
Transformer: Two coils on an iron core that step AC voltage up or down; the current changes the other way.See it in No. 009: How does an inverter or UPS work? →
Transformer: Two coils on an iron core that change an AC voltage in the ratio of their turns.See it in No. L07: What is Faraday's law of induction? →
Transistor: A tiny silicon switch, turned on or off by a voltage on its gate.See it in No. 074: How does a computer work? →
Transistor: A switch with no moving parts, turned on and off by a voltage.See it in No. 073: How does a calculator work? →
Triangulation: Finding a point in 3D from where rays from two or more cameras cross.See it in No. 063: How does motion capture work? →
TRPV1: A heat and pain sensor on nerve endings that opens above about 43 °C and with capsaicin from chillies.See it in No. 034: How does the tongue taste? →
Tungsten carbide: A very hard material used for most ballpoint balls because it barely wears down.See it in No. 002: How does a pen work? →
Turbinates: Three curled, lined shelves on each side wall of the nose that warm, wet and clean the air.See it in No. 033: How does the nose smell? →
Turgor pressure: The push of a plant cell's contents against its wall, about 10 bar, that keeps it firm.See it in No. L14: What are osmosis and diffusion? →
Turnaround: One design drawn from the front, side and back at the same scale, so it can be built.See it in No. 053: How does concept art work? →
Tween: In-betweens calculated by the computer between two keyframes.See it in No. 058: How does 2D animation work? →
Two's complement: Subtracting by adding: flip every bit of the number and add 1.See it in No. 073: How does a calculator work? →

U

Ultrafiltration: Filtering through pores about 0.01 µm wide: it catches bacteria and cysts but not dissolved salts.See it in No. 010: How does an RO water purifier work? →
Umami: The savoury taste of glutamate, found in tomatoes, dal, broth and cheese; named by Kikunae Ikeda in 1908.See it in No. 034: How does the tongue taste? →
Unani: From “Yunani”, Greek: a system of medicine built on Greek humoral theory, developed in the Islamic world and now practised mainly in South Asia.See it in No. 082: How does Unani medicine work? →
Unbalance: Mass that is not spread evenly round a spinning part, so it pulls the machine round in a circle.See it in No. 016: How does a mixer grinder work? →
Unison: Two or three strings tuned to the same note and struck together by one hammer.See it in No. 006: How does a piano work? →
Universal motor: A series-wound motor with brushes and a commutator that runs on AC or DC. It is small, light and very fast.See it in No. 016: How does a mixer grinder work? →
Urea: A harmless molecule the liver makes from toxic ammonia so nitrogen can leave in urine.See it in No. 044: How does the excretory system work? →
Urea cycle: A loop of reactions in liver cells that joins ammonia and carbon dioxide into urea.See it in No. 044: How does the excretory system work? →
Urea cycle: A loop of reactions in liver cells that turns toxic ammonia into harmless urea.See it in No. 037: How does the liver work? →
Urinary tract: Kidneys, ureters, bladder and urethra: the system that makes, stores and releases urine.See it in No. 044: How does the excretory system work? →
UTF-8: The common way to store Unicode text, using 1 to 4 bytes per character.See it in No. 074: How does a computer work? →
UV dose: UV light intensity multiplied by time, in mJ/cm². It sets how many germs are inactivated.See it in No. 010: How does an RO water purifier work? →
UV layout: The mesh cut and flattened so a 2D picture can be painted on it and wrapped back.See it in No. 060: How does 3D animation work? →

V

Vaccine: A harmless version or piece of a germ that trains immune memory.See it in No. 049: How does the immune system work? →
Vagus nerve: Cranial nerve X, which runs from the brainstem to the heart, lungs and gut.See it in No. 045: How does the nervous system work? →
Value: How light or dark a colour is, ignoring its hue.See it in No. 053: How does concept art work? →
Vanishing point: The point where parallel lines seem to meet in a picture drawn in perspective.See it in No. 053: How does concept art work? →
Vasodilation: Blood vessels widening, which sends more blood to the skin, as in a hot bath.See it in No. 085: How does naturopathy work? →
Vectorscope: A round scope showing hue as angle and saturation as distance from the centre.See it in No. 066: How does colour grading work? →
Velocity: Speed in a stated direction. It changes when either the speed or the direction changes.See it in No. L15: What are speed, velocity and acceleration? →
Venturi: A tube that narrows and widens gently; the pressure drop in its neck measures or uses the flow.See it in No. L02: What is Bernoulli's principle? →
Villi: Tiny finger-like bumps lining the small intestine that make its surface huge.See it in No. 043: How does the digestive system work? →
Villi and microvilli: Tiny fingers on the gut lining, and even tinier ones on each lining cell, that multiply its surface.See it in No. 036: How do the intestines work? →
Vis medicatrix naturae: 'The healing power of nature': the belief that the body tends to heal itself.See it in No. 085: How does naturopathy work? →
Viscosity: A fluid's internal friction, which turns some flow energy into heat.See it in No. L02: What is Bernoulli's principle? →
Viscosity: How thick a liquid is. Ballpoint ink is about as thick as honey; fountain pen ink is nearly as thin as water.See it in No. 002: How does a pen work? →
Visual effects (VFX): Effects created or added on computers after the shoot.See it in No. 056: How do practical special effects work? →
Vital capacity: The most air you can breathe out after breathing in as far as you can, about 4 to 5 L.See it in No. 042: How does the respiratory system work? →
Vital capacity: The most air you can breathe out after breathing in as far as you can.See it in No. 029: How do the lungs work? →
Voice band: The 300 to 3,400 Hz range a classic phone line carries.See it in No. 070: How does a telephone work? →
Voltage: The energy each coulomb of charge carries, the push that drives current, measured in volts (V = J/C).See it in No. L27: What are electric current and voltage? →
Voltage: The electrical push that drives charge round a circuit, measured in volts (V).See it in No. L01: What is Ohm's law? →
Vortex: A swirling mass of liquid whose surface dips in the middle and rises at the edge.See it in No. 016: How does a mixer grinder work? →

W

Water activity (aw): How available the water in a food is, from 1.0 down. Most bacteria need more than 0.91.See it in No. L14: What are osmosis and diffusion? →
Watt (W): The unit of power: one joule every second.See it in No. L19: What are work and power? →
Watt-hour: Energy: volts × amp-hours. 1,000 Wh is 1 kWh, one unit on an electricity bill.See it in No. L27: What are electric current and voltage? →
Wave speed: How fast the pattern travels: v = f × λ. Set by the medium, not the source.See it in No. L11: What are waves and resonance? →
Waveform: A scope plotting the brightness of every column of the picture from 0 to 100.See it in No. 066: How does colour grading work? →
Waveguide: A hollow metal duct that carries microwaves from the magnetron to the cooking cavity.See it in No. 008: How does a microwave oven work? →
Wavelength: The length of one wave: λ = c / f, 43 cm at 700 MHz and 8.6 cm at 3.5 GHz.See it in No. 071: How does a mobile phone work? →
Wavelength: The length of one cycle of a wave: its speed divided by its frequency. About 12.2 cm in a microwave oven.See it in No. 008: How does a microwave oven work? →
Wavelength (λ): The distance from one crest to the next.See it in No. L11: What are waves and resonance? →
Weight: A learned number that says how much one input counts, and in which direction.See it in No. 075: How do neural networks work? →
Weight: The pull of gravity on a mass, W = m × g, in newtons. Mass stays the same everywhere.See it in No. L21: What is gravity? →
Weight: The pull of gravity on a mass, W = m g, in newtons. It changes from place to place; mass does not.See it in No. L17: What is inertia? →
Weight: The pull of gravity on a mass: W = m × g, about 9.81 N for each kilogram on Earth.See it in No. L16: What is a force? →
Wet multi-plate clutch: A stack of friction and steel plates in the engine oil that joins or separates the engine and gearbox.See it in No. 021: How does a motorcycle work? →
Wet-bulb temperature: A single temperature that combines heat and humidity; it sets the limit for cooling by sweat.See it in No. 040: How does the skin work? →
White balance: The camera setting that decides which colour of light records as white.See it in No. 055: How does film lighting work? →
Window level and width: The range of Hounsfield units a viewer maps from black to white.See it in No. 019: How does a CT scanner work? →
Work: Energy handed over by a force that moves something: force × distance moved along the force.See it in No. L19: What are work and power? →
Work (W): Energy passed on by a push through a distance, like gas pushing a piston.See it in No. L05: What are the laws of thermodynamics? →

X

X-ray: Light with photons thousands of times more energetic than visible light, able to pass through soft materials.See it in No. 018: How does an X-ray machine work? →

Y

Young's modulus: A material's own stiffness: stress ÷ strain. About 200 GPa for steel.See it in No. L09: What is Hooke's law? →

Z

Z-buffer: A depth value per pixel that keeps only the nearest surface when triangles overlap.See it in No. 061: How does CGI rendering work? →
Zeolite: A porous mineral that soaks up water vapour and gives out heat as it does.See it in No. 013: How does a dishwasher work? →
Zymogen: An enzyme made switched off, such as trypsinogen, so it cannot do damage until it reaches the gut.See it in No. 038: How does the pancreas work? →