(1) A System that classifies, according to wavelength, all energy that moves, harmonically, at the constant velocity of light. (2) A continuum that is conventionally broken into arbitrary segments (as ultraviolet, visible, radio). The entire Range of electromagnetic radiation. The Spectrum usually is divided into seven sections. From the longest wavelengths to the shortest: radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma Ray radiation. (3) The range of frequencies and wavelengths emitted by atomic systems.