- atom
- late 15c., as a hypothetical body, the building block of the universe, from L. atomus (especially in Lucretius), from Gk. atomos "uncut," from a- "not" + tomos "a cutting," from temnein "to cut" (see tome). An ancient term of philosophical speculation (in Leucippus, Democritus), revived 1805 by British chemist Dalton. Atom bomb is from 1945 as both a noun and a verb; cf. atomic.