A very small piece or part; a tiny portion or speck. A very small or the smallest possible amount, trace, or degree: not a particle of doubt. Physics A body whose spatial extent and internal motion and structure, if any, are irrelevant in a specific problem. Physics An elementary particle. Physics A subatomic particle. See Table at subatomic particle. Linguistics 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. Linguistics In some systems of grammatical analysis, any of various short function words, including articles, prepositions, and conjunctions. Roman Catholic Church A small piece of a consecrated host. Roman Catholic Church One of the smaller, individual hosts. Archaic A small division or section of something written, such as a clause of a document.