atom: [16] Etymologically, atom means ‘not cut, indivisible’. Greek átomos ‘that which cannot be divided up any further’ was formed from the negative prefix a- ‘not’ and the base *tom- ‘cut’ (source also of English anatomy and tome), and was applied in the Middle Ages not just to the smallest imaginable particle of matter, but also to the smallest imaginable division of time; an hour contained 22,560 atoms.
Its use by classical writers on physics and philosophy, such as Democritus and Epicurus, was sustained by medieval philosophers, and the word was ready and waiting for 19th-century chemists when they came to describe and name the smallest unit of an element, composed of a nucleus surrounded by electrons. => anatomy, tome
atom (n.)
late 15c., as a hypothetical indivisible body, the building block of the universe, from Latin atomus (especially in Lucretius) "indivisible particle," from Greek atomos "uncut, unhewn; indivisible," 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 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.