Abbreviated Hz, and named for Heinrich Hertz, a 19th-century German physicist who first investigate radio waves. Hertz is technically defined as the inverse of the time required for one complete cycle of a wave. Thus, a 10 Hz sine wave takes 1/10 of a second to complete a full cycle. In practice, it is the frequency or number of wave cycles occurring per second. In the audio range this equates to what we perceive as pitch.