An undulation or vibration, a form of movement by which all radiant energy of the electromagnetic spectrum is thought to travel.

The distance in the line of advance of a sinusoidal wave from any one point to the next point of corresponding phase (e.g., the measured distance from one peak to the next peak). Wavelength is the fundamental property of light—the length of the light wave, which determines its color. Common units of measurement (which is usually from crest to crest) are the micron, the nanometer, and (earlier) the angstrom.