noun A very small piece of matter, a fragment; especially, the smallest possible part of something. .mw-parser-output .defdate{font-size:smaller}
noun Any of various physical objects making up the constituent parts of an atom; an elementary particle or subatomic particle.
noun A part of speech that has no inherent lexical definition but must be associated with another word to impart meaning, often a grammatical category: for example, the English word to in a full infinitive phrase (to eat) or O in a vocative phrase (O Canada), or as a discourse marker (mmm).
noun A part of speech which cannot be inflected.
noun In the Roman Catholic church, a crumb of consecrated bread; also the smaller breads used in the communion of the laity.