noun

1. the act or process of radiating; specif., the process in which energy in the form of rays of light, heat, etc. is sent out through space from atoms and molecules as they undergo internal change
2. the rays sent out; radiant energy
3. radial arrangement of parts
4. Biol. adaptive radiation
5. Nuclear Physics energy emitted as electromagnetic waves, as gamma or X-rays, or as energetic nuclear particles, as neutrons, alpha and beta particles, etc.

Etymology: L radiatio