ra·​di·​a·​tion | \ˌrā-dē-ˈā-shən \

Definition of radiation 

1a : the action or process of radiating

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

2a : something that is radiated

b : energy radiated in the form of waves or particles

3 : radial arrangement

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Other Words from radiation

radiational \ ˌrā-​dē-​ˈā-​shnəl , -​shə-​nᵊl \ adjective
radiationless \ ˌrā-​dē-​ˈā-​shən-​ləs \ adjective
radiative \ ˈrā-​dē-​ˌā-​tiv \ adjective

Examples of radiation in a Sentence

She was exposed to high levels of radiation. He goes in for radiation next week. the sun's radiation of heat
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Recent Examples on the Web

All objects lose heat in the form of infrared radiation, which is invisible to the human eye. Angela Chen, The Verge, "How solar panels could cool our homes while harvesting energy," 12 Nov. 2018 After the retrieval, Mehl went through four rounds of chemotherapy, daily bouts of radiation, and a lumpectomy. Kayla Webley Adler, Marie Claire, "When Your Dreams of Motherhood Are Destroyed," 1 Oct. 2018 There are other possible explanations for this acceleration, like magnetic interaction with the solar wind, pressure from solar radiation, and forces of drag and friction. Chelsea Gohd, Space.com, "Interstellar Visitor 'Oumuamua Is a Comet After All," 27 June 2018 In 2012, 90% of men who had their diagnosis confirmed with a biopsy were quickly ushered into surgery or treated with radiation, despite growing evidence that many of these cancers would never become dangerous. Melissa Healy, latimes.com, "Experts have new advice on prostate cancer screening. Here's why they put it back on the table," 8 May 2018 Bella Simons had brain surgery for two benign but dangerous tumors in 2015, followed by radiation, which caused her to have a stroke two years later. Maureen C. Gilmer, Indianapolis Star, "Carmel teen survives brain tumors and stroke; now she will give back to Riley before another risky surgery," 8 Jan. 2018 This distortion would cause the pulsar radiation to arrive at a slightly different time than expected. Mike Wall, NBC News, "Einstein's theory of relativity passes its toughest test yet," 5 July 2018 The resulting radiation is focused into intense beams, in this case high-energy X-rays, which travel through each hutch. Henrik Knudsen, Smithsonian, "Buried by the Ash of Vesuvius, These Scrolls Are Being Read for the First Time in Millennia," 26 June 2018 The radiation contains no clue as to what went into the black hole. Marcus Woo, Scientific American, "Gravitational “Echoes” Could Reveal Colliding Wormholes," 19 June 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'radiation.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of radiation

15th century, in the meaning defined at sense 1a

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Statistics for radiation

Last Updated

19 Nov 2018

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Time Traveler for radiation

The first known use of radiation was in the 15th century

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English Language Learners Definition of radiation

: a type of dangerous and powerful energy that is produced by radioactive substances and nuclear reactions

medical : the use of controlled amounts of radiation for the treatment of diseases (such as cancer)

: energy that comes from a source in the form of waves or rays you cannot see


ra·​di·​a·​tion | \ˌrā-dē-ˈā-shən \

Kids Definition of radiation

1 : the process of giving off energy in the form of waves or particles

2 : something that is radiated (as light or X-rays)


ra·​di·​a·​tion | \ˌrād-ē-ˈā-shən \

Medical Definition of radiation 

1 : energy radiated in the form of waves or particles

2a : the action or process of radiating with radiation of the pain there may be tenderness over the sciatic nerve— J. A. Key

b(1) : the process of emitting radiant energy in the form of waves or particles

(2) : the combined processes of emission, transmission, and absorption of radiant energy

3 : a tract of nerve fibers within the brain especially : one concerned with the distribution of impulses arising from sensory stimuli to the relevant coordinating centers and nuclei the optic radiations

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