Deutsch: Strahlung / Español: Radiación / Português: Radiação / Français: Rayonnement / Italiano: Radiazione

In physics, radiation is a process in which energetic particles or energetic waves travel through a vacuum, or through matter-containing media that are not required for their propagation. Waves of a mass filled medium itself, such as water waves or sound waves, are usually not considered to be forms of "radiation" in this sense.

In an environmental context, "radiation" refers to the emission of energy as electromagnetic waves or as moving subatomic particles, especially high-energy particles that cause ionization. Radiation can be naturally occurring or can be produced by human activities such as nuclear reactions and the use of certain technologies.

Examples of radiation in the environmental context include:

• Cosmic radiation: the naturally occurring radiation that comes from the sun and other sources in outer space
• Natural background radiation: the low levels of radiation that are present in the environment due to natural sources such as rocks, soil, and the air
• Nuclear radiation: the radiation produced by nuclear reactions, including those from nuclear power plants, weapons, and medical uses
• Electromagnetic radiation: the radiation produced by electrical and magnetic fields, including those from technology such as cell phones, Wi-Fi, and power lines

Exposure to high levels of radiation can be harmful to human health and the environment.