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Physics.

the process in which energy is emitted as particles or waves.
the complete process in which energy is emitted by one body, transmitted through an intervening medium or space, and absorbed by another body.
the energy transferred by these processes.

the act or process of radiating.
something that is radiated.
radial arrangement of parts.
Contemporary Examples

The safety team checked the level of radiation that the firefighters got exposed to, and picked up some people by turns.
A Japanese Firefighter Talks Surviving the Nuclear Reactors Lennox Samuels April 2, 2011

But first, a general primer on radiation measures related to the Japan disaster.
Everyday Radiation in Your Life The Daily Beast March 17, 2011

radiation levels near one reactor were 400 times the normal amount a human should experience in a year.
Rory Kennedy and the Campaign to Get Rid of Nuclear Plants Rory Kennedy, Keven McAlester March 13, 2011

Most can be removed surgically, or zapped with an intense blast of radiation to stop their growth.
Sheryl Crow’s Meningioma, an Exceedingly Common Brain Tumor Daniel Stone June 5, 2012

At the main interior entrance, a man brandished a meter that checked the new arrivals for radiation as they registered.
Japan’s Prime Minister Vows to Rebuild Lennox Samuels March 17, 2011

Historical Examples

He had little fear of being picked up by radiation detectors, thanks to his absorber.
The Happy Man Gerald Wilburn Page

Perhaps the tape was fuzzy or it may have been fogged in transit by radiation.
Mezzerow Loves Company Floyd L. Wallace

There was supposed to be a kind of radiation or infection passing from one nervous system to another.
Human Personality and its Survival of Bodily Death Frederick W. H. Myers

There is also a significant amount of radiation characteristic of uranexite.
Masters of Space Edward Elmer Smith

Physicists did not take long to discover that the radiation from radium was very like the radiation in a “Crookes tube.”
The Outline of Science, Vol. 1 (of 4) J. Arthur Thomson

noun
(physics)

the emission or transfer of radiant energy as particles, electromagnetic waves, sound, etc
the particles, etc, emitted, esp the particles and gamma rays emitted in nuclear decay

(med) Also called radiation therapy. treatment using a radioactive substance
(anatomy) a group of nerve fibres that diverge from their common source
See adaptive radiation
the act, state, or process of radiating or being radiated
(surveying) the fixing of points around a central plane table by using an alidade and measuring tape
n.

mid-15c., “act or process of radiating,” from Middle French radiation and directly from Latin radiationem (nominative radiatio) “a shining, radiation,” noun of action from past participle stem of radiare “to beam, shine, gleam; make beaming,” from radius “beam of light; spoke of a wheel” (see radius). Meaning “rays or beams emitted” is from 1560s. Meaning “divergence from a center” is 1650s.

radiation ra·di·a·tion (rā’dē-ā’shən)
n.

The act or condition of diverging in all directions from a center.

The emission and propagation of energy in the form of rays or waves.

The energy radiated or transmitted in the form of rays, waves, or particles.

A stream of particles or electromagnetic waves that is emitted by the atoms and molecules of a radioactive substance as a result of nuclear decay.

Radiotherapy.

The radial arrangement of anatomical or histological parts.

The spread of a group of organisms into new habitats.

radiation
(rā’dē-ā’shən)

Streams of photons, electrons, small nuclei, or other particles. Radiation is given off by a wide variety of processes, such as thermal activity, nuclear reactions (as in fission), and by radioactive decay.

The emission or movement of such particles through space or a medium, such as air. See Notes at conduction, electromagnetic radiation.

The use of such energy, especially x-rays, in medical diagnosis and treatment.

Energy sent out in the form of particles or waves. (See alpha radiation, beta radiation, blackbody, cosmic rays, electromagnetic radiation, fluorescence, gamma radiation, photon, and quanta.)