The SI unit of frequency, equal to one cycle per second, is the hertz.
The hertz has the symbol Hz and expresses how often a periodic event repeats. One hertz means one complete cycle or periodic event per second, so its formal SI expression is s⁻¹. Frequency is the reciprocal of a cycle’s period: a signal with a period of two seconds has a frequency of 0.5 Hz.
The unit is named after the German physicist Heinrich Rudolf Hertz, who provided conclusive experimental evidence for electromagnetic waves in the late nineteenth century. The name was adopted internationally in the twentieth century, replacing the older phrase “cycles per second.”
Hertz is used for sound, radio signals, alternating current, mechanical vibration, and digital clocks. Common multiples include kilohertz, megahertz, and gigahertz. A frequent mix-up is confusing frequency with angular frequency: a frequency of f hertz corresponds to angular frequency 2πf radians per second. Hertz is also reserved for periodic phenomena; random radioactive events are measured in becquerels.