The distance between two adjacent crests of a transverse wave is called the wavelength.
Wavelength is a property of a wave that can be spotted and used as a way of telling waves apart. The parts of the wave that point up are termed crests while any part that is sloping called a trough.
Wavelength is measured in meters. There is no definite spot from which to start on a wave to measure wavelength. The most popular way of measuring wavelength is from crest to the next crest but can also be measured just as accurately from trough to trough.