The second itself is defined by a frequency measured in hertz
One hertz means one repeating event per second, so a ticking clock runs at 1 Hz and a human heart might beat at roughly 1.2. But the relationship runs both ways. The official second is defined by counting 9,192,631,770 cycles of radiation from a caesium atom, which makes that atomic frequency exact by definition.
The unit honours Heinrich Hertz, the German physicist who first conclusively demonstrated electromagnetic waves and died in 1894 aged just 36. The International Electrotechnical Commission proposed the name in 1935, and the General Conference on Weights and Measures adopted it in 1960, retiring the older cycles per second and its awkward multiples such as megacycles. Newspapers only began making the switch in the late 1960s. As with other units named after people, the symbol Hz is capitalised but the word hertz is not.
Hertz applies only to regular, repeating events. Random ones, such as radioactive decays, are counted in becquerels, even though both reduce to one per second. Similarly, a disc spinning at 60 revolutions per minute has a rotation frequency of 1 Hz, but its angular velocity is expressed in radians per second, not hertz.
The scale is staggering. Babies can hear sounds from 20 Hz up to 20,000, while the average adult's upper limit drops to about 16,000. Radio works in kilohertz, megahertz and gigahertz, the last known as microwaves, and visible light oscillates at around 400 to 790 terahertz. Gravitational waves are hunted by LIGO between 30 and 7,000 Hz and by pulsar timing arrays at mere nanohertz, with planned space detectors such as LISA meant to fill the gap.
For most people, the unit appears on computer specifications. Processor clock speeds climbed from about 1 MHz in late-1970s Atari, Commodore and Apple machines to as high as 6 GHz in IBM Power chips. Critics warn that clock rate is an easily gamed benchmark, since some chips need several cycles per operation while others complete multiple operations in one.
Source: Hertz
