The Collaborative International Dictionary of English v.0.48:

Particle \Par"ti*cle\, n. [L. particula, dim. of pars, gen
   partis, a part: cf. F. particule. See Part, and cf.
   Parcel.]
   1. A minute part or portion of matter; a morsel; a little
      bit; an atom; a jot; as, a particle of sand, of wood, of
      dust.
      [1913 Webster]

            The small size of atoms which unite
            To make the smallest particle of light. --Blackmore.
      [1913 Webster]

   2. Any very small portion or part; the smallest portion; as,
      he has not a particle of patriotism or virtue.
      [1913 Webster]

            The houses had not given their commissioners
            authority in the least particle to recede.
                                                  --Clarendon.
      [1913 Webster]

   3. (R. C. Ch.)
      (a) A crumb or little piece of consecrated host.
      (b) The smaller hosts distributed in the communion of the
          laity. --Bp. Fitzpatrick.
          [1913 Webster]

   4. (Gram.) A subordinate word that is never inflected (a
      preposition, conjunction, interjection); or a word that
      can not be used except in compositions; as, ward in
      backward, ly in lovely.
      [1913 Webster]

   4. (Physics) An elementary particle.
      [PJC]

WordNet (r) 3.0 (2006):

particle
    n 1: (nontechnical usage) a tiny piece of anything [syn: atom,
         molecule, particle, corpuscle, mote, speck]
    2: a body having finite mass and internal structure but
       negligible dimensions [syn: particle, subatomic particle]
    3: a function word that can be used in English to form phrasal
       verbs

Moby Thesaurus II by Grady Ward, 1.0:

148 Moby Thesaurus words for "particle":
   K-meson, ace, adjectival, adjective, adverb, adverbial,
   adversative conjunction, atom, attributive, beta particle, bit,
   butt, chip, chunk, clip, clipping, collop, conjunction,
   conjunctive adverb, coordinating conjunction, copulative,
   copulative conjunction, correlative conjunction, crumb, cut,
   cutting, dab, damn, disjunctive, disjunctive conjunction, dole,
   dollop, dot, dram, dribble, driblet, drop, droplet, dwarf,
   electron, end, exclamatory noun, farthing, fleck, flyspeck,
   form class, form word, fragment, function class, gerundive, gleam,
   gnat, gob, gobbet, grain, granule, graviton, groat, hair, handful,
   hint, hoot, hunk, interjection, iota, jot, little, little bit,
   lota, lump, meson, mesotron, microbe, microorganism, midge, minim,
   minimum, minutia, minutiae, mite, modicum, moiety, molecule,
   morsel, mote, neutron, nutshell, ounce, paring, part of speech,
   participle, past participle, pebble, perfect participle, photon,
   piece, pinch, pinhead, pinpoint, pion, pittance, point,
   preposition, present participle, proton, quark, rasher, ray,
   scintilla, scoop, scrap, scruple, shard, shaving, shiver, shred,
   slice, sliver, smidgen, smidgin, smitch, smithereen, snack, snap,
   snatch, snip, snippet, spark, speck, splinter, spoonful, spot,
   stitch, stump, subordinating conjunction, suggestion, suspicion,
   syllable, tatter, thimbleful, tiny bit, tittle, trifling amount,
   trivia, vanishing point, verbal adjective, whit, whoop