1. (symbol: f) the number of occurrences of a phenomenon, particularly a categorical variable such as sex. For example, it is often of interest to find the frequencies or counts of the men and women who are participating in a research study. See also relative frequency.

2. the number of repetitions of a periodic waveform in a given unit of time. In acoustics, the frequency of a pure tone is the number of cycles of a sinusoidal pressure variation that occur in 1 second. The standard measure of frequency is the hertz (Hz); this replaces, and is equivalent to, cycles per second (cps). For complex periodic waveforms, that is, those consisting of more than one frequency component (e.g., a square wave), the “frequency” is the rate of repetition of the waveform and is more appropriately called the fundamental frequency (or fundamental tone, or simply the fundamental). The period of a waveform is the time to complete one repetition and is the reciprocal of the frequency (or fundamental frequency). For waveforms that are not periodic, such as white noise, the frequency is undefined. In such cases, the spectral characteristics should be described. See also sound spectrum.