English compound of wave + length, formalized around 1850 after Young's wave theory of light proved that light has measurable spatial periodicity.
The distance between successive crests of a wave, determining its frequency and energy in electromagnetic radiation.
Compound of 'wave' (from Old English wafian 'to wave, fluctuate') + 'length' (from Old English lengþu). The concept was established by Thomas Young's wave theory of light in the early 1800s, but the compound word 'wavelength' was first recorded around 1850. The figurative sense 'on the same wavelength' (sharing an understanding) appeared by 1927. Key roots: *webʰ- (Proto-Indo-European: "to weave, move to and fro"), *dlongʰos (Proto-Indo-European: "long").