In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium.[1][2] This includes electro-magnetic radiation such as radio waves, visible light, and x-rays, particle radiation such as α, β, and neutron radiation and acoustic radiation such as ultrasound, sound, and seismic waves. Radiation may also refer to the energy, waves, or particles being radiated.

Radiation is often categorized as either ionizing or non-ionizing depending on the energy of the radiated particles. Ionizing radiation carries more than 10 eV, which is enough to ionize atoms and molecules, and break chemical bonds. This is an important distinction due to the large difference in harmfulness to living organisms. A common source of ionizing radiation is radioactive materials that emit α, β, or γ radiation, consisting of helium nuclei, electrons or positrons, and photons, respectively. Other sources include X-rays from medical radiography examinations and muons, mesons, positrons, neutrons and other particles that constitute the secondary cosmic rays that are produced after primary cosmic rays interact with Earth's atmosphere.