1. energy transmitted in the form of waves, such as electromagnetic radiation (e.g., heat, light, microwaves, short radio waves, ultraviolet rays, X-rays), or in the form of a stream of nuclear particles (e.g., alpha particles, beta particles, gamma rays, electrons, neutrons, protons). Such waves or particles are used for diagnostic, therapeutic, or experimental purposes (see radiation therapy).

2. in neuroscience, the spread of excitation to adjacent neurons.

3. more generally, the outward spread of something from a central point.