The SI unit of frequency is the hertz, abbreviated Hz.
One hertz means one cycle or periodic event per second, so its expression in SI base units is s⁻¹. Frequency measures how often a repeating phenomenon occurs. It is the reciprocal of period: a signal with a period of 0.5 seconds has a frequency of 2 Hz.
The unit honors German physicist Heinrich Rudolf Hertz, who provided decisive experimental evidence for electromagnetic waves in the late nineteenth century. The General Conference on Weights and Measures adopted hertz as an SI unit in 1960, replacing the older wording “cycles per second” in formal usage.
Hertz is used for sound, radio signals, alternating current, vibrations, and electronic clock rates. Prefixes indicate powers of ten: 1 kHz equals 1,000 Hz, 1 MHz equals one million Hz, and 1 GHz equals one billion Hz. Frequency should not be confused with angular frequency, which is measured in radians per second, or with decibels, which express a ratio or level.