Greek ἄτομος (atomos), "uncuttable"; Latin primordia, corpora prima, semina rerum (Lucretius)
The indivisible bodies of which everything is made. Atoms are infinite in number, unchangeable, and eternal; they differ only in shape, size, and weight, and they are always in motion. Everything we perceive, including the soul, is a temporary combination of atoms moving through void. See The Universe Operates Naturally Through Atoms and Space.
1. Key Texts
Quote from Epicurus, Letter to Herodotus 41Moreover, among bodies, some are compounds, and others those from which compounds are made. And these latter are indivisible and unchangeable - if, that is, all things are not going to be destroyed into what is not, but are going to remain firmly in being during the dissolution of compounds - full in nature, and incapable of dissolution at any point or in any way. Therefore the principles of bodies must be the indivisible natures. (tr. Long & Sedley)