The hertz (symbol: Hz) is a measure of frequency, informally defined as the number of events occurring per second. It is the basic unit of frequency in the International System of Units (SI), and is used worldwide in both general-purpose and scientific contexts. Hertz can be used to measure any periodic event; the most common uses for hertz are to describe radio and audio frequencies, more or less sinusoidal contexts in which case a frequency of 1 Hz is equal to one cycle per second.
More formally, the unit hertz is defined by the International System of Units (SI) such that the hyperfine splitting in the ground state of the caesium 133 atom is exactly 9 192 631 770 hertz, ν (hfs Cs) = 9 192 631 770 Hz.. Equivalently, 1 Hz = 1⁄9,192,631,770 ν (hfs Cs). This definition is derived from the SI definition of the second. hertz are inverse seconds, s-1. In practice, the hertz simply replaced the older cycle per second.
In English, hertz is used as both singular and plural. As any SI unit, Hz can be prefixed; commonly used multiples are kHz (kilohertz, 103 Hz), MHz (megahertz, 106 Hz), GHz (gigahertz, 109 Hz) and THz (terahertz, 1012 Hz). One hertz simply means one cycle per second (typically that which is being counted is a complete cycle); 100 Hz means one hundred cycles per second, and so on. The unit may be applied to any periodic event—for example, a clock might be said to tick at 1 Hz, or a human heart might be said to beat at 1.2 Hz. Neither the cycle per second nor the Hertz, however, are regularly used in nonsinusoidal contexts. The "frequency" (activity) of aperiodic or stochastic events, especially radioactive decay, is expressed in becquerels.
