frequency definition. In physics, the number of crests of a wave that move past a given point in a given unit of time. The most common unit offrequency is the hertz (Hz), corresponding to one crest per second. Thefrequency of a wave can be calculated by dividing the speed of the wave by the wavelength.amplitude, in physics, the maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium position. It is equal to one-half the length of the vibration path.