radiation

1. Direct transmission of heat or electromagnetic energy through space or another medium. It is the 'non-ionizing' type of radiation that does not affect chemical bonds and thus is usually harmless. Also called radiant energy.

2. Particulate emanation of unstable or split atomic nuclei, such as protons or helium nuclei (alpha radiation), high-energy electrons (beta radiation), and neutrons (from fission). It is the 'ionizing' type of radiation that directly breaks down chemical bonds and is likely to be harmful.