The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   3. Anything extremely small; a particle; a whit.
      [1913 Webster]

            There was not an atom of water.       --Sir J. Ross.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Atom \At"om\, v. t.
   To reduce to atoms. [Obs.] --Feltham.
   [1913 Webster] Atom bomb

WordNet (r) 3.0 (2006):

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: atom,
       molecule, particle, corpuscle, mote, speck]

Moby Thesaurus II by Grady Ward, 1.0:

175 Moby Thesaurus words for "atom":
   Bohr atom, Dirac atom, I, Lewis-Langmuir atom, Thomson atom, ace,
   acid, acidity, agent, air, alkali, alkalinity, alloisomer, anion,
   antacid, atomic model, atomic particles, atoms, base, biochemical,
   bit, brute matter, building block, cation, chemical,
   chemical element, chromoisomer, component, compound, constituent,
   copolymer, cubical atom, dab, dash, dimer, dole, dot, dram,
   dribble, driblet, dwarf, dyad, earth, electron, element,
   elementary particle, elementary unit, fabric, farthing, fire,
   fleck, flyspeck, fragment, fundamental particle, gobbet, grain,
   granule, groat, hair, handful, heavy chemicals, heptad, hexad,
   high polymer, homopolymer, hydracid, hyle, hypostasis,
   inorganic chemical, ion, iota, isobar, isomer, isotopic isobar,
   jot, labeled atom, little, little bit, lota, macromolecule,
   material, material world, materiality, matter, medium, meson,
   metamer, minim, minimum, minutiae, mite, modicum, molecule, monad,
   monomer, mote, natural world, nature, neutral atom, neutralizer,
   no other, nonacid, none else, normal atom, nothing else,
   nought beside, nuclear atom, nuclear isomer, nuclear particle,
   nuclide, nutshell, octad, one, one and only, organic chemical,
   ounce, oxyacid, particle, pebble, pentad, physical world, pinch,
   pittance, planetary shell, plenum, point, polymer, proton,
   pseudoisomer, quark, radiation atom, radical, reagent, recoil atom,
   scruple, shade, shell, smack, smidgen, smitch, soupcon, speck,
   spice, spoonful, spot, stripped atom, stuff, subshell, substance,
   substratum, suggestion, sulfacid, suspicion, tagged atom, tangible,
   tetrad, the four elements, thimbleful, tincture, tinge, tiny bit,
   tittle, touch, trace, tracer, tracer atom, triad, trifling amount,
   trimer, trivia, unit, unit of being, valence shell, water, whit