1. High-energy radiation capable of producing ionization in substances through which it passes.

It includes non-particulate radiation; such as. X-rays, and radiation produced by energetic charged particles; such as, alpha and beta rays, and by neutrons, as from a nuclear reaction.

2. Particulate or electromagnetic radiation that produces ionization in a medium through which it passes.
3. Any radiation; such as, a stream of alpha particles or x-rays, that produces ionization as it passes through a medium.
4. Particles or photons that have sufficient energy to produce ionization directly in their passage through a substance.
5. Particles that are capable of nuclear interactions in which sufficient energy is released to produce ionization.
6. Photons of high-energy electromagnetic radiation and particle forms of radiation that have sufficient energy to produce ions by removing electrons from atoms or molecules.