Wavelength
The distance between one wave crest of light and the next, which largely decides the colour we see.
Also: nanometre
Light travels as waves. The length of each wave is tiny and is measured in nanometres, billionths of a metre. Human eyes respond to wavelengths from about 380 to about 750 nanometres. Short waves in that range look violet and blue, middle ones green and yellow, long ones orange and red. Most light we meet is a mixture of many wavelengths, and the colour we see depends on the blend. Some colours, such as magenta and brown, do not match any single wavelength at all.