Wavelength

The distance between two successive points on a periodic wave that are in phase — typically crest to crest or trough to trough. Denoted λ (lambda), measured in metres.

For a sinusoidal wave y = A sin(kx − ωt), the wavelength is λ = 2π/k, where k is the wavenumber. It combines with frequency f to give the phase velocity v = fλ, so in a medium where v is fixed (e.g. sound in air, light in vacuum), choosing a wavelength determines the frequency and vice versa.