means The distance between two corresponding points of a wave — crest to crest or trough to trough — that determines a wave's color, pitch, or character.

from A transparent compound of "wave" and "length," assembled by 19th-century physicists as the wave theory of light and sound took hold and demanded a name for that crest-to-crest span. "Wave" traces back to Old English wafian, to move to and fro, while "length" comes from Old English lengþu, kin to "long." The figurative sense — being "on the same wavelength" as someone — is a 20th-century gift from radio, where two sets must tune to the same wavelength to hear each other at all.