particle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A very small piece or part; a tiny portion or speck.
• noun A very small or the smallest possible amount, trace, or degree.
• noun A body whose spatial extent and internal motion and structure, if any, are irrelevant in a specific problem.
• noun An elementary particle.
• noun A subatomic particle.
• noun An uninflected item that has grammatical function but does not clearly belong to one of the major parts of speech, such as up in He looked up the word or to in English infinitives.
• noun In some systems of grammatical analysis, any of various short function words, including articles, prepositions, and conjunctions.
• noun Ecclesiastical A portion or fragment of the Eucharistic host.
• noun Archaic A small part of something written, such as a clause of a document.

from The Century Dictionary.

• noun A small part or piece, especially a small part or portion of some material substance: as, a particle of dust.
• noun Specifically, any very small piece or part of anything: absolutely, a minute quantity; anything very small; an atom; a bit: as, he has not a particle of patriotism or virtue; are you fatigued? Not a particle.
• noun In grammar, a part of speech that is considered of minor consequence, or that plays a subordinate part in the structure of the sentence, as connective, sign of relation, or the like: such are especially conjunctions, prepositions, and the primitive adverbs. The term is loose and unscientific.
• noun Synonyms and Particle, Atom, Molecule, Corpuscle,iota, jot, mite, tittle, whit, grain, scrap, shred, scin-tilla. Atom and molecule are exact scientific terms; the other two of the italicized words are not. A particle is primarily a minute part or piece of a material substance, or, as in the case of dust, pollen, etc., a substance that exists in exceedingly minute form. Corpuscle is a somewhat old word for particle, to which it has almost entirely yielded place, taking up instead a special meaning in physiology. See definitions; see also part, n.
• noun In a document of any kind, a very small part of any statement or proposition; a clause.
• noun In the Roman Catholic Church, the host given to each lay communicant.
• noun In mech., a body or portion of matter so minute that, while it possesses mass, it may be treated as a geometrical point.

from the GNU version of the Collaborative International Dictionary of English.

• noun A minute part or portion of matter; a morsel; a little bit; an atom; a jot.
• noun Any very small portion or part; the smallest portion.
• noun A crumb or little piece of consecrated host.
• noun The smaller hosts distributed in the communion of the laity.
• noun (Gram.) A subordinate word that is never inflected (a preposition, conjunction, interjection); or a word that can not be used except in compositions.
• noun (Physics) An elementary particle.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (nontechnical usage) a tiny piece of anything
• noun a function word that can be used in English to form phrasal verbs
• noun a body having finite mass and internal structure but negligible dimensions