Definition of: atom

(at′əm) noun
1. Chem. The smallest part of an element capable of existing alone or in combination and which cannot be changed or destroyed in any chemical reaction: an atom of sulfur, carbon, etc.: distinguished from molecule.
2. Physics The electrically neutral combination of a nucleus and its complement of electrons to form a relatively stable, distinguishable unit, regarded as a unitary system.
3. A hypothetical entity admitting of no division into smaller parts.
4. An exceedingly small quantity or particle; iota. [<L atomus <Gk. atomos indivisible <a- not + temnein cut]