Frequency

Frequency is the number of times that a periodic function repeats the same sequence of values during a unit variation of the independent variable;
the number of complete alternations per second of an alternating current;
the number of complete oscillations per second of energy (such as sound or electromagnetic radiation) in the form of waves.

The SI unit for frequency is the hertz (Hz). One hertz is the same as one cycle per second.

Sources:

League of SI Superheroes – Professor Second.