Meaning

noun (physics and chemistry) the smallest component of an element having the chemical properties of the element

noun (nontechnical usage) a tiny piece of anything

originFrom Middle English attome, from Middle French athome, from Latin atomus (“smallest particle”), from Ancient Greek ἄτομος (átomos, “indivisible”), from ἀ- (a-, “not”) + τέμνω (témnō, “to cut”, o-grade in τομ-) + -ος (-os). Atoms are so named because they were historically thought up as to be the smallest unit of matter, and thus indivisible. Doublet of atomus.

Etymology from Wiktionary, CC BY-SA 3.0