Frequency

Expressed in Hertz (Hz), frequency is the number of oscillations of a periodic phenomenon per unit of time.

This is normally a time frequency f, related to the period T (expressed in s) of the observed phenomenon using the following formula: f = 1/T.

It also possible to speak of spatial frequency, analogous to time frequency, generally an angular distance, a measure of how often a sinusoidal component repeats per unit distance.