Abbreviated "Hz," one Hertz is equal to one cycle per second. In 1883, Heinrich Hertz detected electromagnetic waves, and his name was adopted to measure the number of electromagnetic waves, or cycles, in a signal.
Hertz is widely used to refer to the clock rate of a CPU; for example, 2 GHz means two billion cycles per second. The term is also used for other repeating cycles such as frame rate; for example, a 60 Hz TV displays 60 frames per second. See MHz
The Computer's Heart
Coincidentally, Heinrich Hertz's last name actually means "heart" in German, and the clock is like the computer's heart. It activates everything by pumping pulses into the circuits (see clock
A 2.25 MHz Clock
The first commercial computer in the 1950s, the UNIVAC I's CPU, which you could literally walk into, had a 2.25 MHz clock that generated 2.25 million pulses per second. Today's computer clocks are a thousand times faster. (Image courtesy of Deutsches Museum, Munich, Archives, R2931.)