radiation |ˌrādēˈā sh ən|noun1 Physics the emission of energy as electromagnetic waves or as moving subatomic particles, esp. high-energy particles that cause ionization.• the energy transmitted in this way, as heat, light, electricity, etc.2 chiefly Biology divergence out from a central point, in particularevolution from an ancestral animal or plant group into a variety of new forms.DERIVATIVESradiational |-ˈā sh ənl| adjectiveradiationally |-ˈā sh ənl-ē| adverbORIGIN late Middle English (denoting the action of sending out rays of light): from Latin radiatio(n-), from radiare ‘emit rays’ (see radiate ).