The number of complete oscillations or cycles of a periodic quantity occurring in unit time. The frequency is related to the angular frequency ω by the relation ω = 2πν. The frequency of a periodic quantity, such as an alternating current, is given by the number of times the quantity passes through its zero value in the same sense in unit time.

The frequency of electromagnetic radiation is related to the wavelength, λ, by the equation ν = c/λ, where c is the speed of light.