- Date: 15th century
- b : the process of emitting radiant energy in the form of waves or particles
- c (1) : the combined processes of emission, transmission, and absorption of radiant energy (2) : the transfer of heat by radiation — compare conduction, convection
- 2 a : something that is radiated
- 3 : radial arrangement
In physics, radiation describes any process in which energy travels through a medium or through space, ultimately to be absorbed by another body. Non-physicists often associate the word ionizing radiation (e.g., as occurring in nuclear weapons, nuclear reactors, and radioactive substances), but it can also refer to electromagnetic radiation (i.e., radio waves, infrared light, visible light, ultraviolet light, and X-rays) which can also be ionizing radiation, to acoustic radiation, or to other more obscure processes. What makes it radiation is that the energy radiates (i.e., it travels outward in straight lines in all directions) from the source. This geometry naturally leads to a system of measurements and physical units that are equally applicable to all types of radiation.