Wavelength
The distance (measured in meters, or fractions of a meter) from one peak to the next in a wave. When light with different distributions of wavelengths enters our eye, different colours are perceived by our visual system.
Representation of waves with longer wavelengths (top) and shorter wavelengths (bottom). The wavelength is the measured distance from one peak to the next, for a particular wave.
The light wave on the top has longer wavelength and lower energy than the light wave on the bottom.
A common misconception about light is that the wavelengths are somehow coloured. See https://colourliteracy.org/light-rays-not-coloured for clarification of this misconception.
