IGCSE Physics Definitions Sheet: All the Key Terms You Need

Physics at IGCSE requires you to define terms with scientific precision. Many exam questions begin with “Define…” or “State what is meant by…” — and full marks depend on using the exact phrasing examiners expect.

This IGCSE Physics definitions sheet covers every essential term from the Cambridge IGCSE Physics (0625/0972) syllabus, organised by topic for quick reference and revision.

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1. Measurement and Units

Term	Definition
Scalar quantity	A quantity that has magnitude (size) only (e.g. speed, mass, energy).
Vector quantity	A quantity that has both magnitude and direction (e.g. velocity, force, acceleration).
SI units	The international system of units used in science (e.g. metre, kilogram, second, ampere, kelvin).
Accuracy	How close a measured value is to the true value.
Precision	How close repeated measurements are to each other.
Parallax error	An error in reading a scale caused by viewing it from an angle rather than straight on.

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2. Motion

Term	Definition
Speed	The distance travelled per unit time.
Velocity	The speed of an object in a given direction; the rate of change of displacement.
Acceleration	The rate of change of velocity, measured in m/s².
Deceleration	A negative acceleration — a decrease in velocity over time.
Distance	The total length of the path travelled by an object (scalar).
Displacement	The straight-line distance from the starting point to the finishing point in a specified direction (vector).
Terminal velocity	The constant maximum velocity reached when the driving force equals the resistive force (e.g. air resistance equals weight).
Free fall	Motion under the influence of gravity only, with no air resistance.
Acceleration due to gravity (g)	The acceleration of an object falling freely near the Earth’s surface, approximately 9.8 m/s² (often taken as 10 m/s²).

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3. Forces

Term	Definition
Force	A push or pull that can change the shape, speed or direction of motion of an object, measured in newtons (N).
Mass	The amount of matter in an object, measured in kilograms (kg); a scalar quantity.
Weight	The gravitational force acting on an object; W = mg, measured in newtons (N).
Gravitational field strength (g)	The force per unit mass acting on an object in a gravitational field, measured in N/kg.
Friction	A force that opposes the motion of one surface over another.
Air resistance (drag)	A frictional force that opposes the motion of an object through air.
Resultant force	The single force that has the same effect as all the forces acting on an object combined.
Newton’s first law	An object remains at rest or continues to move at constant velocity unless acted on by a resultant force.
Newton’s second law	The resultant force on an object is equal to its mass multiplied by its acceleration: F = ma.
Newton’s third law	For every action there is an equal and opposite reaction — forces always occur in pairs on different objects.
Moment	The turning effect of a force about a pivot: moment = force × perpendicular distance from the pivot.
Centre of gravity	The point at which the entire weight of an object appears to act.
Equilibrium	A state where the resultant force and the resultant moment on an object are both zero.
Hooke’s law	The extension of a spring is directly proportional to the applied force, up to the limit of proportionality.
Limit of proportionality	The point beyond which the extension of a spring is no longer proportional to the force applied.

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4. Momentum

Term	Definition
Momentum	The product of an object’s mass and velocity: p = mv, measured in kg m/s.
Impulse	The change in momentum of an object, equal to the force multiplied by the time for which it acts: impulse = FΔt.
Conservation of momentum	In a closed system, the total momentum before an event equals the total momentum after the event.

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5. Energy, Work and Power

Term	Definition
Energy	The capacity to do work, measured in joules (J).
Kinetic energy	The energy an object has due to its motion: KE = ½mv².
Gravitational potential energy	The energy an object has due to its position in a gravitational field: GPE = mgh.
Work done	The energy transferred when a force moves an object through a distance in the direction of the force: W = Fd.
Power	The rate at which energy is transferred or work is done: P = W/t, measured in watts (W).
Efficiency	The ratio of useful energy output to total energy input, often expressed as a percentage.
Conservation of energy	Energy cannot be created or destroyed — it can only be transferred from one form to another.
Renewable energy	Energy from sources that are naturally replenished and will not run out (e.g. solar, wind, hydroelectric).
Non-renewable energy	Energy from sources that will eventually run out because they are consumed faster than they are formed (e.g. fossil fuels, nuclear fuel).

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6. Thermal Physics

Term	Definition
Temperature	A measure of the average kinetic energy of the particles in a substance.
Internal energy	The total kinetic and potential energy of all the particles in a substance.
Specific heat capacity	The amount of energy required to raise the temperature of 1 kg of a substance by 1 °C, measured in J/(kg °C).
Specific latent heat	The energy required to change the state of 1 kg of a substance without changing its temperature.
Latent heat of fusion	The energy required to change 1 kg of a solid to a liquid at its melting point.
Latent heat of vaporisation	The energy required to change 1 kg of a liquid to a gas at its boiling point.
Conduction	The transfer of thermal energy through a material by the vibration and collision of particles, without the particles themselves moving from place to place.
Convection	The transfer of thermal energy through a fluid by the circulation of currents caused by differences in density.
Radiation	The transfer of thermal energy by electromagnetic waves (infrared), requiring no medium.
Thermal expansion	The increase in size of a substance when its temperature increases, due to increased particle vibration.

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7. Waves

Term	Definition
Wave	A disturbance that transfers energy from one place to another without transferring matter.
Transverse wave	A wave in which the vibrations are perpendicular to the direction of energy transfer (e.g. light, water waves).
Longitudinal wave	A wave in which the vibrations are parallel to the direction of energy transfer (e.g. sound).
Amplitude	The maximum displacement of a point on a wave from its rest (equilibrium) position.
Wavelength (λ)	The distance between two consecutive points in phase on a wave (e.g. crest to crest).
Frequency (f)	The number of complete waves passing a point per second, measured in hertz (Hz).
Period (T)	The time taken for one complete wave to pass a point: T = 1/f.
Wave speed	The speed at which a wave travels: v = fλ.
Reflection	The bouncing back of a wave when it hits a boundary between two media.
Refraction	The change in direction of a wave as it passes from one medium to another due to a change in speed.
Diffraction	The spreading of a wave as it passes through a gap or around an obstacle.
Total internal reflection	The complete reflection of a wave inside a denser medium when the angle of incidence exceeds the critical angle.
Critical angle	The angle of incidence above which total internal reflection occurs.

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8. Sound

Term	Definition
Sound	A longitudinal wave caused by vibrations, requiring a medium to travel through.
Pitch	The perceived highness or lowness of a sound, determined by its frequency.
Loudness	The perceived volume of a sound, determined by its amplitude.
Echo	A reflected sound wave heard after a delay.
Ultrasound	Sound with a frequency above 20 000 Hz — above the range of human hearing.

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9. Light and the Electromagnetic Spectrum

Term	Definition
Electromagnetic spectrum	The continuous range of electromagnetic waves ordered by frequency or wavelength, from radio waves to gamma rays.
Electromagnetic wave	A transverse wave that can travel through a vacuum at 3 × 10⁸ m/s, consisting of oscillating electric and magnetic fields.
Normal	An imaginary line perpendicular to a surface at the point where a ray meets the surface.
Angle of incidence	The angle between the incoming ray and the normal.
Angle of reflection	The angle between the reflected ray and the normal.
Law of reflection	The angle of incidence equals the angle of reflection.
Refractive index	The ratio of the speed of light in a vacuum to the speed of light in a medium: n = c/v.
Converging lens	A lens that brings parallel rays of light to a focus (convex lens).
Diverging lens	A lens that spreads parallel rays of light apart (concave lens).
Focal length	The distance from the centre of a lens to its principal focus.
Real image	An image formed where rays of light actually converge; can be projected onto a screen.
Virtual image	An image formed where rays of light appear to come from; cannot be projected onto a screen.

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10. Electricity

Term	Definition
Electric current (I)	The rate of flow of electric charge: I = Q/t, measured in amperes (A).
Charge (Q)	A property of matter that causes it to experience a force in an electric field, measured in coulombs (C).
Potential difference (V)	The energy transferred per unit charge passing between two points: V = W/Q, measured in volts (V).
Resistance (R)	The opposition to the flow of current: R = V/I, measured in ohms (Ω).
Ohm’s law	The current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant.
Electromotive force (e.m.f.)	The energy transferred per unit charge by a source of electrical energy (e.g. battery), measured in volts.
Series circuit	A circuit in which components are connected end-to-end so there is only one path for current.
Parallel circuit	A circuit in which components are connected across each other so there are multiple paths for current.
Electrical power	The rate of energy transfer by an electrical device: P = IV, measured in watts (W).
Kilowatt-hour (kWh)	A unit of energy equal to the energy used by a 1 kW device in 1 hour.

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11. Magnetism and Electromagnetism

Term	Definition
Magnetic field	The region around a magnet or current-carrying conductor where a magnetic force is experienced.
Magnetic field lines	Lines that show the direction and strength of a magnetic field; they go from north to south outside the magnet.
Electromagnet	A magnet made by passing an electric current through a coil of wire, often with an iron core.
Motor effect	The force experienced by a current-carrying conductor placed in a magnetic field.
Fleming’s left-hand rule	A rule used to predict the direction of the force on a current-carrying conductor in a magnetic field (thumb = force, first finger = field, second finger = current).
Electromagnetic induction	The generation of an electromotive force (e.m.f.) in a conductor when it moves through a magnetic field or when a magnetic field changes around it.
Transformer	A device that changes the voltage of an alternating current using electromagnetic induction between two coils wound on an iron core.
Direct current (d.c.)	Current that flows in one direction only.
Alternating current (a.c.)	Current that repeatedly reverses direction.

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12. Nuclear Physics

Term	Definition
Nucleus	The dense, positively charged centre of an atom containing protons and neutrons.
Radioactivity	The spontaneous and random emission of radiation from an unstable nucleus.
Alpha particle (α)	A particle consisting of 2 protons and 2 neutrons (a helium nucleus), emitted during radioactive decay; strongly ionising, weakly penetrating.
Beta particle (β)	A high-speed electron emitted from the nucleus when a neutron converts to a proton; moderately ionising and penetrating.
Gamma ray (γ)	A high-frequency electromagnetic wave emitted from the nucleus; weakly ionising, highly penetrating.
Half-life	The time taken for half the radioactive nuclei in a sample to decay, or for the activity of a source to halve.
Background radiation	Low-level radiation that is always present in the environment from natural and artificial sources.
Nuclear fission	The splitting of a large, unstable nucleus into two smaller nuclei, releasing energy and neutrons.
Nuclear fusion	The joining of two small nuclei to form a larger nucleus, releasing a large amount of energy.
Chain reaction	A self-sustaining series of fission reactions where neutrons released from one fission event trigger further fission events.

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13. Space Physics

Term	Definition
Galaxy	A large collection of billions of stars held together by gravity.
Solar system	The Sun and all the objects that orbit it, including planets, moons, asteroids and comets.
Orbit	The curved path of an object around a larger object due to gravity.
Gravitational force	The attractive force between any two objects with mass.
Red-shift	The increase in wavelength (shift towards the red end of the spectrum) of light from galaxies moving away from us — evidence that the universe is expanding.

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Exam Tips for IGCSE Physics Definitions

1. Learn definitions word-for-word — “rate of change of velocity” is accepted for acceleration; “how fast something speeds up” is not.
2. Include units — when defining a quantity, always state the SI unit (e.g. “measured in newtons”).
3. Know the difference between mass and weight — mass is measured in kg and does not change; weight is measured in N and depends on gravitational field strength.
4. Scalar vs vector — examiners love asking you to classify quantities. Speed is scalar; velocity is vector.
5. Use equations as definitions — stating W = Fd along with “energy transferred when a force moves through a distance” strengthens your answer.

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Last updated: March 2026. Based on the Cambridge IGCSE Physics (0625/0972) syllabus.

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