Particle

noun

1. A very small piece of matter, a fragment; especially, the smallest possible part of something.
2. Any of various physical objects making up the constituent parts of an atom; an elementary particle or subatomic particle.
3. A word that has a particular grammatical function but does not obviously belong to any particular part of speech, such as the word to in English infinitives or O as a vocative particle.
4. A part of speech which cannot be inflected: an adverb, preposition, conjunction or interjection.
5. In the Roman Catholic church, a crumb of consecrated bread; also the smaller breads used in the communion of the laity.