[2] Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals (sound), radio waves, and light.

For cyclical phenomena such as oscillations, waves, or for examples of simple harmonic motion, the term frequency is defined as the number of cycles or repetitions per unit of time.

The conventional symbol for frequency is f or ν (the Greek letter nu) is also used.

It was adopted by the CGPM (Conférence générale des poids et mesures) in 1960, officially replacing the previous name, cycle per second (cps).

Some commonly used conversions are listed below: For periodic waves in nondispersive media (that is, media in which the wave speed is independent of frequency), frequency has an inverse relationship to the wavelength, λ (lambda).

When monochromatic waves travel from one medium to another, their frequency remains the same—only their wavelength and speed change.

Measurement of frequency can be done in the following ways: Calculating the frequency of a repeating event is accomplished by counting the number of times that event occurs within a specific time period, then dividing the count by the period.

An old method of measuring the frequency of rotating or vibrating objects is to use a stroboscope.

The strobe light is pointed at the rotating object and the frequency adjusted up and down.

A downside of this method is that an object rotating at an integer multiple of the strobing frequency will also appear stationary.

Cyclic processes that are not electrical, such as the rotation rate of a shaft, mechanical vibrations, or sound waves, can be converted to a repetitive electronic signal by transducers and the signal applied to a frequency counter.

Current research is extending this method to infrared and light frequencies (optical heterodyne detection).

Visible light is an electromagnetic wave, consisting of oscillating electric and magnetic fields traveling through space.

Even higher-frequency waves are called X-rays, and higher still are gamma rays.

where c is the speed of light (c in vacuum or less in other media), f is the frequency and λ is the wavelength.

Sound propagates as mechanical vibration waves of pressure and displacement, in air or other substances.

[10] In general, frequency components of a sound determine its "color", its timbre.

When speaking about the frequency (in singular) of a sound, it means the property that most determines its pitch.

[12] In many media, such as air, the speed of sound is approximately independent of frequency, so the wavelength of the sound waves (distance between repetitions) is approximately inversely proportional to frequency.

In Europe, Africa, Australia, southern South America, most of Asia, and Russia, the frequency of the alternating current in household electrical outlets is 50 Hz (close to the tone G), whereas in North America and northern South America, the frequency of the alternating current in household electrical outlets is 60 Hz (between the tones B♭ and B; that is, a minor third above the European frequency).

Aperiodic frequency is the rate of incidence or occurrence of non-cyclic phenomena, including random processes such as radioactive decay.

[14] It is defined as a rate, f = N/Δt, involving the number of entities counted or the number of events happened (N) during a given time duration (Δt);[citation needed] it is a physical quantity of type temporal rate.