The rate of oscillation in a sound (or other) wave in a specified time period. More technically, it is the number of times that a wave repeats itself in one second, which is expressed in hertz (Hz) or cycles per second (e.g., middle C has a frequency of 256 Hz, and alpha waves as measured EEG 8-10 Hz). High-pitch sounds have a high fundamental frequency (F₀).
See Dynamics, Electroencephalogram (EEG), FM system, Fundamental frequency (F₀), Pitch