From WordNet (r) 2.0 (August 2003) [wn]:
atom
n 1: (physics and chemistry) the smallest component of an element
having the chemical properties of the element
2: (nontechnical usage) a tiny piece of anything [syn:
molecule, particle, corpuscle, mote, speck]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Atom \At"om\, n. [L. atomus, Gr. ?, uncut, indivisible; 'a priv.
+ ?, verbal adj. of ? to cut: cf. F. 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.
[1913 Webster]
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.
[1913 Webster]
2. (Chem.) The smallest particle of matter that can enter
into combination; one of the elementary constituents of a
molecule.
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3. Anything extremely small; a particle; a whit.
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There was not an atom of water. --Sir J. Ross.
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From The Collaborative International Dictionary of English v.0.48 [gcide]:
Atom \At"om\, v. t.
To reduce to atoms. [Obs.] --Feltham.
[1913 Webster] Atom bomb