The opposite of a crest is called a trough. This is the point at which the maximum value of negative or downward movement is displayed.
Amplitude is the measurement from the resting point of a wave to either the very top of the wave or the very bottom of the wave.
The wavelength of a wave is the completion of one cycle of a wave. The actions of a wave are repeated in cycles. This is called a wave cycle.Learn more about Optics & Waves