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