Source : Webster's Revised Unabridged Dictionary (1913)

Atom \At"om\, v. t.
   To reduce to atoms. [Obs.] --Feltham.

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.

   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.

   3. Anything extremely small; a particle; a whit.

            There was not an atom of water.       --Sir J. Ross.

Source : WordNet®

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}]