From 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.
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2. Any very small portion or part; the smallest portion; as,
he has not a particle of patriotism or virtue.
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The houses had not given their commissioners
authority in the least particle to recede.
--Clarendon.
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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.
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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.
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4. (Physics) An elementary particle.
[PJC]