Wavelength
A wavelength is the length of the shortest repeating part of a "sine wave". All waves can be formed by adding up sine waves. That is, every wave is a total of sine waves, which may be identified by Fourier analysis. It is also known as the distance between a crest and another crest of a wave (highest point above equilibrium) or the distance between a trough and another trough of a wave (lowest point below the equilibrium of a wave).