Wave Length of a transverse wave is the distance between two successive crests or two successive troughs. Or, it is twice the distance between a trough and a crest.
Wave length can be calculated as the distance between two points along the wave motion, which have the same phase (same displacement value).
We can measure it by knowing the speed of a transverse wave and dividing it by the frequency of the wave. If a wave is set up in a string or rope tied to a wall, then the wavelength can be calculated from the length of the string or rope.