Yes, WAVELENGTH is a Scrabble word!

Letter values in Scrabble US: W 4, A 1, V 4, E 1, L 1, E 1, N 1, G 2, T 1, H 4.

Is it a valid word?

WAVELENGTH is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the distance (measured in the direction of propagation) between two points in the same phase in consecutive cycles of a wave
a shared orientation leading to mutual understanding