wavelength is the measurement form crest to crest it's the distance between successive crests of a wave, especially points in a sound wave or electromagnetic wave.
The trough of a wave is the lowest point
the crest of a wave is the highest point of a wave
a point on a vibrating body or wave measured from its equilibrium position. It is equal to one-half the length of the vibration path. ... When the amplitude of a wave steadily decreases because its energy is being lost, it is said to be damped.