The **inverse second**, **reciprocal second**, or **per second** (s^{−1}) is a unit of frequency, defined as the multiplicative inverse of the second (a unit of time). It is dimensionally equivalent to:

- the unit hertz – the SI unit for cycles per second
- the unit becquerel – the SI unit for aperiodic or stochastic
*radionuclide*events per second - the unit baud – the unit for symbol rate over a communication link
- strain rate – the velocity gradient (comprising the shear rate and directional expansion rate) of a fluid or solid

It also provides the denominator for temporal rates, such as that of angular frequency in radians per second.

*"The SI unit of frequency is given as the hertz, implying the unit cycles per second; the SI unit of angular velocity is given as the radian per second; and the SI unit of activity is designated the becquerel, implying the unit counts per second. Although it would be formally correct to write all three of these units as the reciprocal second, the use of the different names emphasises the different nature of the quantities concerned."*"Units with special names and symbols; units that incorporate special names and symbols".*"(d) The hertz is used only for periodic phenomena, and the becquerel (Bq) is used only for stochastic processes in activity referred to a radionuclide."*"BIPM - Table 3". BIPM. Retrieved 2012-10-24.