The act or process of radiating: the radiation of heat and light from a fire. Physics Emission and propagation and emission of energy in the form of rays or waves. Physics Energy radiated or transmitted as rays, waves, in the form of particles. Physics A stream of particles or electromagnetic waves emitted by the atoms and molecules of a radioactive substance as a result of nuclear decay. The act of exposing or the condition of being exposed to such energy. The application of such energy, as in medical treatment. Anatomy Radial arrangement of parts, as of a group of nerve fibers connecting different areas of the brain. The spread of a group of organisms into new habitats. Adaptive radiation.