Word parts wave + length Pronunciation (UK) IPA (key): /ˈweɪvlɛŋ(k)θ/ (US) IPA (key): /ˈweɪvˌlɛŋ(k)θ/ Audio (US)(file) Noun Singular wavelength Plural wavelengths (countable) The physical length of a wave, especially an electromagnetic wave. For electromagnetic waves, the wavelength is approximately the speed of light divided by frequency. Wikiwand - on Seamless Wikipedia browsing. On steroids.