How to use this list
- Learn the exact wording. Most definitions follow a pattern: a quantity “per unit” something (speed, density, pressure, power).
- Learn the equation alongside the words where there is one.
- Cover the Definition column and test yourself one topic at a time.
- Definitions marked * are not worded in the syllabus (it only asks you to define them), so a standard textbook definition is given.
- Source: Cambridge IGCSE Physics 0625 syllabus for 2026–2028 (PDF). For the equations, see the IGCSE Physics formula sheet.
Motion, forces and energy
| Term | Definition |
|---|---|
| Speed | Distance travelled per unit time. |
| Velocity | Speed in a given direction. |
| Acceleration Extended | Change in velocity per unit time. |
| Deceleration Extended | A negative acceleration. |
| Scalar quantity Extended | A quantity with magnitude (size) only, e.g. distance, speed, time, mass, energy, temperature. |
| Vector quantity Extended | A quantity with magnitude and direction, e.g. force, weight, velocity, acceleration, momentum. |
| Mass | A measure of the quantity of matter in an object at rest relative to the observer. |
| Weight | A gravitational force on an object that has mass. |
| Gravitational field strength | Force per unit mass (g = W/m); near the Earth it is about 9.8 N/kg. |
| Density | Mass per unit volume (ρ = m/V). |
| Spring constant Extended | Force per unit extension (k = F/x). |
| Limit of proportionality Extended | The point beyond which the extension is no longer proportional to the load.* |
| Solid friction | The force between two surfaces that may impede motion and produce heating. |
| Moment of a force | A measure of its turning effect: force × perpendicular distance from the pivot. |
| Equilibrium | When there is no resultant force and no resultant moment on an object. |
| Centre of gravity | The point through which the whole weight of an object appears to act.* |
| Momentum Extended | Mass × velocity (p = mv). |
| Impulse Extended | Force × time for which the force acts (FΔt = Δ(mv)). |
| Resultant force Extended | The change in momentum per unit time (F = Δp/Δt). |
| Work done | Equal to the energy transferred (W = Fd = ΔE). |
| Power | Work done per unit time, or energy transferred per unit time. |
| Efficiency Extended | (Useful energy output ÷ total energy input) × 100%, or the same with power. |
| Pressure | Force per unit area (p = F/A). |
Thermal physics
| Term | Definition |
|---|---|
| Absolute zero | The lowest possible temperature (−273 °C), where particles have the least kinetic energy. |
| Brownian motion | The random motion of microscopic particles in a suspension, caused by random collisions with the particles of the gas or liquid. |
| Specific heat capacity Extended | The energy required per unit mass per unit temperature increase (c = ΔE/(mΔθ)). |
| Evaporation | The escape of more-energetic particles from the surface of a liquid. |
| Thermal radiation | Infrared radiation, emitted by all objects; it does not need a medium. |
Waves
| Term | Definition |
|---|---|
| Wave | Waves transfer energy without transferring matter. |
| Wavelength | The distance between two neighbouring identical points on a wave, e.g. crest to crest.* |
| Frequency | The number of complete waves passing a point per second, measured in hertz (Hz).* |
| Amplitude | The maximum displacement of a point on the wave from its rest position.* |
| Transverse wave | The direction of vibration is at right angles to the direction of propagation. |
| Longitudinal wave | The direction of vibration is parallel to the direction of propagation. |
| Critical angle | The angle of incidence in the denser medium for which the angle of refraction is 90°.* |
| Refractive index Extended | The ratio of the speeds of a wave in two different regions (n = sin i / sin r). |
| Principal focus | The point where rays parallel to the principal axis meet after passing through a converging lens.* |
| Focal length | The distance from the centre of the lens to the principal focus.* |
| Virtual image | Formed when diverging rays are extrapolated backwards; it does not form a visible projection on a screen. |
| Monochromatic light Extended | Visible light of a single frequency. |
| Echo | The reflection of sound waves. |
| Ultrasound | Sound with a frequency higher than 20 kHz. |
Electricity and magnetism
| Term | Definition |
|---|---|
| Magnetic field | A region in which a magnetic pole experiences a force. |
| Electric field Extended | A region in which an electric charge experiences a force. |
| Electric current Extended | The charge passing a point per unit time (I = Q/t). |
| Electromotive force (e.m.f.) | The electrical work done by a source in moving a unit charge around a complete circuit. |
| Potential difference (p.d.) | The work done by a unit charge passing through a component. |
| Resistance | The ratio of p.d. to current (R = V/I). |
| Kilowatt-hour (kWh) | The energy transferred by a 1 kW appliance in 1 hour (3.6 MJ).* |
| Conventional current Extended | Flows from positive to negative; free electrons flow from negative to positive. |
Nuclear and space physics
| Term | Definition |
|---|---|
| Proton number (Z) | The number of protons in the nucleus.* |
| Nucleon number (A) | The total number of protons and neutrons in the nucleus.* |
| Isotopes | Atoms of the same element with the same number of protons but different numbers of neutrons.* |
| Background radiation | Ionising radiation that is always present in the environment, from natural and artificial sources.* |
| Radioactive decay | A spontaneous and random change in an unstable nucleus that can emit α-particles, β-particles and/or γ-radiation. |
| Half-life | The time taken for half the nuclei of an isotope in any sample to decay. |
| Light-year | The distance travelled in space by light in one year (about 9.5 × 10¹⁵ m). |
| Redshift | An increase in the observed wavelength of electromagnetic radiation from receding stars and galaxies. |
| Hubble constant Extended | The ratio of a galaxy's speed away from the Earth to its distance from the Earth (H₀ = v/d). |