1. (λ‎) The distance on a wave between successive points that are in phase, e.g. for a water wave, the wavelength is the distance from one crest to the next. Wavelength is related to the velocity (v) and frequency (f) by: λ‎ = v/f. The inverse of wavelength is wavenumber.

2. Of a fold system, the distance between one hinge or trough and the next. It is rarely possible, or necessary, to measure the wavelengths of folds precisely.