What is the distance between two peaks of a wave called?

The distance between two peaks of a wave is called its wavelength.

More precisely, wavelength is the distance between matching points on successive cycles of a wave. For a simple transverse wave, that usually means crest to crest or trough to trough. It can also be measured from one identical zero crossing to the next, provided the wave is moving in the same direction at both points.

Wavelength is represented by the Greek letter lambda (λ) and is commonly measured in metres. It is related to wave speed and frequency by the equation v = fλ: at a fixed speed, higher-frequency waves have shorter wavelengths. This is why blue light, which has a higher frequency than red light, has a shorter wavelength.

A common mix-up is confusing wavelength with amplitude. Wavelength measures horizontal spacing in the wave pattern; amplitude measures the size or height of the disturbance. In sound, wavelength describes the spacing of pressure variations, even though the wave itself is longitudinal rather than visibly crest-shaped.