Radiation is defined as the emission of energy from an atom in the form of a wave or particle. Such energy is released as electromagnetic radiation or as radioactivity. Electromagnetic radiation includes radio waves, infrared waves or heat, visible light, ultraviolet radiation, x rays, gamma rays, and cosmic rays. Radioactivity, emitted when an atomic nucleus undergoes decay, usually takes the form of a particle such as an alpha particle or beta particle, though atomic decay can also release electromagnetic gamma rays.

While radiation in the form of heat, visible light, and even ultraviolet light is essential to life, the word "radiation" is often used to refer only to those emissions which can damage or kill living things. Such harm is specifically attributed to radioactive particles as well as the electromagnetic rays with frequencies higher than visible light (ultraviolet, x rays, gamma rays). Harmful electromagnetic radiation is...