noun

1. Obsolete any of the indivisible particles postulated by philosophers as the basic component of all matter
2. a tiny particle of anything; jot
3. Chem., Physics any of the smallest particles of an element that combine with similar particles of other elements to produce compounds: atoms combine to form molecules, and consist of a complex arrangement of electrons revolving about a positively charged nucleus containing (except for hydrogen) protons and neutrons and other particles

Etymology: ME attome < OFr atome < L atomus < Gr atomos, uncut, indivisible, atom < a-, not + tomos < temnein, to cut: see -tomy