How frequency is measured?

Answer 1

Frequency is the number of wavelengths produced in one second.

Answer 2

In the simplest sense, frequency is a tally of the number of times something happens. Usually, it is scaled to a unit of time, as in, for example, the frequency of automobile accidents in the U.S. during a year.

In a more specific sense, frequency is often used to measure waveforms; for examples, light or pressure in a medium (usually--but not always--called "sound"). In this case, frequency is generally defined as the number of complete waves or cycles per second. This unit of measurement is called "Hertz," abbreviated Hz.

Note that waveforms in nature are usually combinations of many different frequencies.