Etymology
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atom (n.)

late 15c., as a hypothetical indivisible extremely minute body, the building block of the universe, from Latin atomus (especially in Lucretius) "indivisible particle," from Greek atomos "uncut, unhewn; indivisible," from a- "not" (see a- (3)) + tomos "a cutting," from temnein "to cut" (from PIE root *tem- "to cut"). An ancient term of philosophical speculation (in Leucippus, Democritus); revived scientifically 1805 by British chemist John Dalton. In late classical and medieval use also a unit of time, 22,560 to the hour. Atom bomb is from 1945 as both a noun and a verb; compare atomic.

updated on July 10, 2017

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Definitions of atom from WordNet

atom (n.)
(physics and chemistry) the smallest component of an element having the chemical properties of the element;
atom (n.)
(nontechnical usage) a tiny piece of anything;
Synonyms: molecule / particle / corpuscle / mote / speck
From wordnet.princeton.edu, not affiliated with etymonline.