Frequency is a measure of the number of occurrences of a repeating event per unit time. For rotations, oscillations or waves, it is defined as a number of cycles per unit of time. To calculate the frequency, the number of occurrences of the event within a fixed time interval are counted, and then it's divided by the length of the time interval. So, Frequency = 1 / Time

In SI system, the unit of frequency is hertz (Hz), named after Heinrich Hertz. For example, 1 Hz means that an event repeats once per second, 2 Hz is twice per second, etc.