Wavelength
From Coastal Wiki
Definition of Wavelength:
Straightline distance between two successive wave crests.^{[1]}.
This is the common definition for Wavelength, other definitions can be discussed in the article

This is the definition for regular sinusoidal waves. Real sea waves are irregular without a welldefined wavelength. The wave field off the coast can be characterized by a wave spectrum. Wavelength is then a statistical parameter, see Statistical description of wave parameters.
References
 ↑ CIRIA (1996). Beach management manual. CIRIA Report 153.