par·ti·cle 
(pärtĭ-kəl)
(pärtĭ-kəl)n.
1. A very small piece or part; a tiny portion or speck.
2. A very small or the smallest possible amount, trace, or degree: not a particle of doubt.
3. Physics
a. A body whose spatial extent and internal motion and structure, if any, are irrelevant in a specific problem.
b. An elementary particle.
c. A subatomic particle.
4. Linguistics
a. 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.
b. In some systems of grammatical analysis, any of various short function words, including articles, prepositions, and conjunctions.
5. Ecclesiastical A portion or fragment of the Eucharistic host.
6. Archaic A small part of something written, such as a clause of a document.
[Middle English, from Latin particula, diminutive of pars, part-, part; see PART.]