1. molecule A particle is a very small part of any material substance; as, a particle of sand or of dust; it is a general term, not accurately determinate in meaning. Atom (Gr. a- privative, not, and te Synonyms: particle, atom, grain, mite, scrap, whit, corpuscle, iota, molecule, shred, element, jot, scintilla, tittle Antonyms: aggregate, entirety, mass, quantity, sum, sum total, total, whole