A frequency (f) is a physical variable which measures how often a event is repeated in a defined period of time.

The SI-unit is Hertz (hz) or s^-1.

A common scenario in which the frequency is used, is in a wave, in which it can be calculated by measuring the Period T (the time it takes to complete one cycle) and using the equation: f = 1/T

Detailed Definition:

Another equation by which the frequency can be calculated, is the following with \( \lambda = \) wavelength and v= phase velocity (if the wave is detected in a vacuum, the following applies: v=c)

\( f = v/ \lambda \).

There exist two special types of frequency, the angular frequency and the spatial frequency, which will be explained in the following:

The angular frequency (ω) describes how fast a vibration / wave is occurring by measuring the overstepped phase angle of the vibration / wave in a defined time period. This frequency can be calculated in the already introduced frequency f by using the following equation:

\( \omega = 2 \pi f = 2 \pi /T \).

The spatial frequency (ξ) measures how often sinusoidal components (components of a sinusoidal wave) of a structure repeat per unit of distance, which can be calculated by using the following equation:

\( \xi = 1/ \lambda = f/v \).

Etymology:

frequency - Latin frequentia (" an assembling in great numbers, a crowding; crowd, multitude, thong")

Sample Sentence(s):

The frequency can be identified by calculating the inverse of the period T.

The frequency is a variable which can characterize a wave.