ra·di·a·tion (rāˌdē-āˈshən)

1. The act or condition of diverging in all directions from a center.
2. The emission and propagation of energy in the form of rays or waves.
3. The energy radiated or transmitted in the form of rays, waves, or particles.
4. A stream of particles or electromagnetic waves that is emitted by the atoms and molecules of a radioactive substance as a result of nuclear decay.
5. Radiotherapy.
6. The radial arrangement of anatomical or histological parts.
7. The spread of a group of organisms into new habitats.