Antinode - a point in a medium with a standing wave where the displacement is momentarily a maximum

Atmosphere - a non-SI unit of pressure

Atomic (or proton) number - the number of protons in a nucleus

Atomic mass - the mass of an atom measured in units of u

Atomic mass unit - a unit of mass equal to of the mass of a neutral atom of carbon-12

Average - another word for mean

Average power - for sinusoidally varying voltages and currents the average power in a conductor is half the peak value

Average speed - the ratio of distance travelled to total time taken

Average velocity - the ratio of displacement to total time taken

Avogadro constant - the number of particles in one mole