Etymology
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radiation (n.)

mid-15c., radiacion, "act or process of emitting light," 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. In modern physics, "emission or transmission of energy in the form of waves or particles," especially in reference to ionizing radiation, from early 20c.

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radiology (n.)
1900, "medical use of X-rays," later extended to "scientific study of radiation," from radio-, combining form of radiation, + Greek-based scientific suffix -ology. Related: Radiological.
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rad (n.)

1918, "x-ray dose unit," a shortened form of radiation (q.v.). The meaning "unit of absorbed dose of ionizing radiation" is by 1954, an acronym from radiation absorbed dose. As shortened form of radical (n.), it is attested in political slang from 1820. Teen slang adjectival sense of "extraordinary, wonderful" is from late 1970s (see radical (adj.)).

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radiant (adj.)

mid-15c., "shining, bright, shooting or emitting diverging rays of light," later also of heat, from Latin radiantem (nominative radians) "beaming, shining," present participle of radiare "to beam, shine" (see radiation). Of beauty, wit, etc., "sparkling, beaming," attested from c. 1500. Related: Radiantly.

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radiance (n.)

c. 1600, "brilliant light, brightness shooting in diverging rays or beams," from radiant (adj.) or else from Medieval Latin radiantia "brightness," from radiare "to beam, shine" (see radiation). Figurative use, of beauty, joy, etc., is by 1761. Related: Radiancy.

Radiance ... is generally a light that is agreeable to the eyes; hence the word is often chosen for corresponding figurative expressions: as, the radiance of his cheerfulness; the radiance of the gospel. Brilliance represents a light that is strong, often too strong to be agreeable, and marked by variation or play and penetration : as, the brilliance of a diamond or of fireworks. [Century Dictionary]
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radioisotope (n.)

"a radioactive isotope," 1946, from radio-, combining form of radiation, + isotope.

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photon (n.)

"unit of electromagnetic radiation," 1926, from photo- "light" + -on "unit." Related: Photonic.

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REM (n.1)

also R.E.M., rem, unit for measuring ionizing radiation, 1947, acronym of roentgen equivalent man.

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maser (n.)

a type of laser that emits microwaves, 1955, acronym from "microwave amplification (by) stimulated emission (of) radiation." Related: Mase (v.).

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Van Allen 
name of radiation belts around the Earth (and certain other planets), 1959, from U.S. physicist James A. Van Allen (1914-2006), who reported them in 1958.
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