wavelength

Wavelength, the length of a wave, is the distance to the point at which such a disturbance in a medium (that constitutes a wave) begins a repeat of itself. This applies to any waves, such as light waves, sound waves, or ocean waves. Wavelength has this relationship with wave speed and frequency:

wavelength × frequency = wave speed

A wave can be characterized by its wavelength or by its frequency: if you are given its frequency (and know the speed of the waves, e.g., the speed of light), the above relationship yields its wavelength, and vice versa. In the case of electromagnetic waves, the same characteristic can also be specified by its energy per photon (photon energy), which is directly related to its frequency. Which of these three is typically used to specify electromagnetic waves varies over its spectrum: Wavelength is typically used to specify this characteristic of infrared and visible light, Radio is more likely to be specified by frequency (with microwave showing less such preference), and X-rays and gamma rays are more likely to be specified by photon energy.