Wavelength:

Light can be thought of as an electromagnetic wave. Like a wave on a string, the electromagnetic wave has a "wavelength", which is the distance it travels in one oscillation. For example, visible light has a wavelength between 400 and 800 nanometres, while infra-red light has a wavelength between 800 nm and around 300 microns. The wavelength of millimetre radiation is, unsurprisinly, around a millimetre, while sub-millimetre light has a wavelength a little under a millimetre.Image credit: Wikipedia Commons