Frequency is the number of waves passing a fixed point in a second. Frequency is measured in hertz (Hz), kilohertz (kHz), or megahertz (MHz), named after after the scientist who first produced and observed radio waves in the laboratory. For example, if 1,000 peaks cross a given point in one second, the frequency is 1,000 cycles per second or 1,000 hertz = 1 kHz. The frequency (f), wavelength (λ), and velocity (v) of a harmonic wave are related by the equation v = f λ.


Musical sounds have frequencies in the range 30–20,000 Hz; alternating-current electricity supply is at 60 Hz in the United States, but usually 50 Hz in the rest of the world. According to quantum theory, the frequency (ν, nu) of electromagnetic radiation provides a measure of the energy (E) of its quanta: E = , where h is the Planck constant.


More generally, frequency is the number of times a value occurs in some time interval.