Wavelength
All electromagnetic waves are defined by their wavelength lambda, representing the spatial periodicity of oscillations (the distance between two oscillation peaks for example). The wavelength, which is also the distance travelled by the wave during one oscillation period, is inversely proportional to the frequency; it is expressed in metres.
The shorter the wavelength, the greater the intensity of the wave and therefore the more energetic it is (X-rays for example, with a wavelength of 10^{-11} m to 10^{-8} m).
The wavelength represents the distance travelled by the wave during one oscillation period.