Frequency is a measurement of the number of occurrences of a repeated event in a given time. To compute frequency, one fixes a time interval, counts the number of occurrences of an event, and divides this count by the length of the time interval. The result is presented in units of hertz (Hz) after German physicist Heinrich Rudolf Hertz, where 1 Hz is an event that occurs once per second.

In measuring the frequency of sound, electromagnetic waves (such as radio or light), electrical signals, or other waves, the frequency in hertz is the number of cycles of the repetitive waveform per second. Frequency has an inverse relationship to the concept of wavelength. The frequency "f" is equal to the speed "v" of the wave divided by the wavelength of the wave.