particle
n//ˈpɑɹtɪkəl//partikel
A very small piece of matter, a fragment; especially, the smallest possible part of something. [from 14th c.]
(physics) Any of various physical objects making up the constituent parts of an atom; an elementary particle or subatomic particle. [from 19th c.]
(grammar) 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).