Frequency from the perspective of musical sounds is a measure of the number of times a peak or trough in the sound wave pass a given point in one second.  It is measured in cycles per second with one cycle per second being referred to as one hertz. It is so named in honour of the 19th-century German physicist Heinrich Rudolf Hertz and abbreviated as Hz.

In musical terms, a standard note to which instruments are tunes at the present time is generally A4 which has a frequency of 440 Hz. In the past, other standards of tuning were in use.