A way energy moves from one place to another. Sometimes waves move materials the way water ripples in a pond move the water. Other times, waves don’t move anything around when they transfer energy. For example, X-rays and other waves on the don’t make any ripples when they move energy from place to place. We classify waves based on how long their wavelengths are. A wavelength is the distance from peak to peak (or valley to valley) of the wave.