in physics, number of waves that pass a fixed point in unit time; also the number of cycles or vibrations undergone during one unit of time by a body in periodic motion (q.v.). A body in periodic motion is said to have undergone one cycle or one vibration after passing through a series of events or positions and returning to its original state. See also angular velocity; simple harmonic motion.

If the period, or time interval, required to complete one cycle or vibration is ¹/₂ second, the frequency is 2 per second; if the period is ¹/₁₀₀ of an hour, the frequency is 100 per hour. In general, the frequency is the reciprocal of the period, or time interval—i.e., frequency = 1/period = 1/(time interval). The frequency with which the Moon revolves about the Earth is slightly more than 12 cycles per year; the frequency of the A string of a violin is 440 vibrations or cycles per second.

The symbols most often used for frequency are f and the Greek letters nu (ν) and omega (ω). Nu is used more often when specifying electromagnetic waves, such as light, X rays, and gamma rays; omega is mostly used by electrical engineers in referring to alternating current. Usually frequency is expressed in the hertz unit, named in honour of the 19th-century German physicist Heinrich Rudolf Hertz, one hertz being equal to one cycle per second, abbreviated Hz; one kilohertz (kHz) is 1,000 Hz, and one megahertz (MHz) is 1,000,000 Hz.

In spectroscopy another unit of frequency, the wave number (q.v.), is sometimes used.