Atom, n.
Etymology: [L. atomus, Gr. atome. See Tome.]
1. (Physics) (a) An ultimate indivisible particle of matter. (b) An ultimate particle of matter not necessarily indivisible; a molecule. (c) A constituent particle of matter, or a molecule supposed to be made up of subordinate particles.
Note: These three definitions correspond to different views of the nature of the ultimate particles of matter. In the case of the last two, the particles are more correctly called molecules. Dana.
2. (Chem.)
The smallest particle of matter that can enter into combination; one of the elementary constituents of a molecule.
1 more senses
3. Anything extremely small; a particle; a whit. There was not an atom of water. Sir J. Ross.
1 other Webster entry under this spelling
Atom, v. t.
To reduce to atoms. [Obs.] Feltham.
Shades of meaning
Fernald has no entry of its own for atom; he lists it under part, particle.
Where he writes about it:
Atom (Gr. a- privative, not, and temno, cut) etymologically signifies that which can not be cut or divided, and is the smallest conceivable particle of matter, regarded as absolutely homogeneous and as having but one set of properties; atoms are the ultimate particles of matter. — from particle
A molecule is made up of atoms, and is regarded as separable into its constituent parts; as used by physicists, a molecule is the smallest conceivable part which retains all the characteristics of the substance; thus, a molecule of water is made up of two atoms of hydrogen and one atom of oxygen. — from particle