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 frequency
- the unit radian per second – the SI unit for angular frequency and angular velocity
- 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
- the unit of strain rate – the velocity gradient (comprising the shear rate and directional expansion rate) of a fluid or solid