Energy that is radiated or transmitted in the form of rays or waves or particles WordNet 3.0 & Open English WordNet

In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. Wikipedia

A process by which energetic particles or energetic waves travel. Wikipedia Disambiguation

Waves or particles propagating through space or through a medium, carrying energy Wikidata

Emission of any rays from either natural or man-made origins, such as radio waves, the sun's rays, medical X-rays and the fall-out and nuclear wastes produced by nuclear weapons and nuclear energy production. Radiation is usually divided between non-ionizing radiation, such as thermal radiation OmegaWiki

Radiation that is capable of energizing atoms sufficiently to remove electrons from them. OmegaWiki

The transfer of energy via radiation (as opposed to convection or conduction). Wiktionary

Transfer of energy via radiation. Wiktionary (translation)