A rhythmic periodic motion.

(Hz) The standard SI unit of frequency.
Definition: The number of repetitions of a periodic process per unit time. It is equal to the old unit cycles or oscillations each second of a simple harmonic motion. The unit is named for the German physicist Heinrich Rudolf Hertz.
Larger units are:
kilohertz (KHz) = 1 000 Hz = 10³ Hertz
megahertz (MHz) = 1 000 KHz = 10⁶ Hertz
gigahertz (GHz) = 1 000 MHz = 10⁹ Hertz
terahertz (THz) = 1 000 GHz = 10¹² Hertz
petahertz (PHz) = 1 000 THz = 10¹⁵ Hertz
exahertz (EHz) = 1 000 PHz = 10¹⁸ Hertz

See also Oscillation, Coherence, Duty Cycle, Cine Mode, and System International.