wavelength

The wavelength is the peak-to-peak distance of a wave. For sea waves this may vary from a few centimetres for surface ripples to many metres for deep sea waves. For sounds this varies from a few centimetres for notes at the top of a piano to several metres for those at the bottom. For light the wavelength is between around 400nm (violet) to 700nm (red).

This is to be contrasted with the frequency, which is the time taken for the wave to move from peak-to-peak at a fixed location. For sound waves the two are related by the speed of sound, for electriomagnetic waves the speed of light:
      wavelength / frequency   =   speed