Etymology
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radioactivity (n.)

"state of being radioactive; emissions from radioactive material or processes," 1899, from French radioactivité, coined 1898 by the Curies; see radioactive.

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unit (n.)

1560s, "single number regarded as an undivided whole," alteration of unity on the basis of digit. Popularized in John Dee's English translation of Euclid, to express Greek monas (Dee says unity formerly was used in this sense). Meaning "single thing regarded as a member of a group" is attested from 1640s. Extended sense of "a quantity adopted as a standard of measure" is from 1738. Sense of "group of wards in a hospital" is attested from 1893.

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curie (n.)

"unit of radioactivity," 1910, named for French physicist Pierre Curie (1859-1906), who, with his wife, Marie (1867-1934), discovered radium. The family name in Old French means "kitchen."

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half-life (n.)

also half life, 1864, "unsatisfactory way of living," from half + life; the sense in physics, "amount of time it takes half a given amount of radioactivity to decay" is first attested 1907.

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photon (n.)

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

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unitary (adj.)

1847, "characterized by unity or uniformity;" 1865, "of or relating to a unit;" see unit + -ary.

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monad (n.)

1610s, "unity, arithmetical unit," 1610s, from Late Latin monas (genitive monadis), from Greek monas "unit," from monos "alone" (from PIE root *men- (4) "small, isolated"). In Leibnitz's philosophy, "an ultimate unit of being, a unit of the universal substance" (1748); he apparently adopted the word from Giordano Bruno's 16c. metaphysics, where it referred to a hypothetical primary indivisible substance at once material and spiritual. Related: Monadic; monadism.

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decay (n.)

mid-15c., "deterioration, decline in value, gradual loss of soundness or perfection," from decay (v.). Obsolete or archaic in reference to fortune or property; meaning "decomposition of organic tissue" is from 1590s. In physics, the meaning "gradual decrease in radioactivity" is by 1897.

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B.T.U. 

1889 as an abbreviation of British Thermal Unit (1862), a commercial unit of electrical energy (the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit); the French Thermal Unit is the amount of heat required to raise 1 kilogram of water 1 degree centigrade. Also from 1889 as an abbreviation of Board of Trade Unit, in electicity "1,000 watt hours."

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Euro (n.)

name for the basic monetary unit of a pan-European currency, from 1996.

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