Radiation generally means the transmission of waves, objects or information from a source into a surrounding medium or destination.

Within physics, related concepts are:

• Ionizing radiation is a stream of particles (photons or other particles) with sufficient energy to cause ionization of atoms or molecules.
• Non-ionizing radiation does not have this sufficient energy.
• Electromagnetic radiation: light is the visible form; thermal radiation is (usually) the heat form. Overall, the range of wavelengths encompasses Extremely Low Frequency waves in the many km range, AM radio, FM radio, TV and microwaves, infrared (thermal) wavelengths, visible light, ultraviolet, X-rays, and gamma rays.
• Gravitational radiation
• Particle radiation is any kind of radiation in which the individual elements behave like particles, for example fast or slow Neutron radiation.
• Cherenkov radiation is the emission of electromagnetic radiation by a charged particle moving through an insulating medium faster than the speed of light in that medium.
• Synchrotron radiation is emitted by charged particles being accelerated in magnetic fields and moving at near the speed of light. This happens, for example, if the particle moves in a circle, as in a synchrotron.

In Biology, adaptive radiation is a process in evolutionary biology by which one species becomes many in order to adapt to specific ecological niches.

Radiation is sometimes also used, inaccurately, to refer to radioactive contamination, the release of radioactive isotopes into the environment. These isotopes then release ionizing radiation, which can be especially severe if the isotopes have been taken up by plants, animals or humans, since the isotopes then release their ionizing radiation from inside the organism.