From AMS Glossary
The ordered sequence of all known electromagnetic radiations, extending from the shortest cosmic rays through gamma rays, x-rays, ultraviolet radiation, visible radiation, infrared radiation, and including microwave and all other radio wavelengths.
The division of this continuum of wavelengths (or frequencies) into a number of named subportions is rather arbitrary and, with one or two exceptions, the boundaries of the several subportions are only vaguely defined. Nevertheless, to each of the commonly identified subportions there correspond characteristic types of physical systems capable of emitting radiation at those wavelengths. Thus, gamma rays are emitted from the nuclei of atoms as they undergo any of several types of nuclear rearrangements; visible light is emitted, for the most part, by atoms with planetary electrons undergoing transitions to lower energy states; infrared radiation is associated with characteristic molecular vibrations and rotations; and radio waves, broadly speaking, are emitted by virtue of the accelerations of free electrons in metals as, for example, the moving electrons in a radio antenna wire.