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.

1545–55; < Latin radiātiōn- (stem of radiātiō) a glittering, shining. See radiate, -ion

radiational, adjective
antiradiation, adjective
interradiation, noun
nonradiation, noun
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
radiation (ˌreɪdɪˈeɪʃən)
1.  physics
 a.  the emission or transfer of radiant energy as particles, electromagnetic waves, sound, etc
 b.  the particles, etc, emitted, esp the particles and gamma rays emitted in nuclear decay
2.  med Also called: radiation therapy treatment using a radioactive substance
3.  anatomy a group of nerve fibres that diverge from their common source
4.  See adaptive radiation
5.  the act, state, or process of radiating or being radiated
6.  surveying the fixing of points around a central plane table by using an alidade and measuring tape

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1555, from L. radiationem (nom. radiatio) "a shining, radiation," noun of action from radiare "to beam, shine," from radius "beam of light" (see radius).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

radiation ra·di·a·tion (rā'dē-ā'shən)

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

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

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

  4. 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.

  5. Radiotherapy.

  6. The radial arrangement of anatomical or histological parts.

  7. The spread of a group of organisms into new habitats.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
radiation  [%PREMIUM_LINK%]     (rā'dē-ā'shən)  Pronunciation Key 
    1. 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.

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

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

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

radiation definition

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.)

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
Eric says that the intensity of cell phone radiation diminishes with distance.
To camp on the moon, astronauts need to be shielded from solar radiation.
Plus, being exposed to cosmic radiation for years and years and years cannot be
  a good lifestyle choice.
Even today these people have horrible health problems and are feeling the
  effects of the radiation.
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