Definition - A catch-all term for function words that do not belong to any of the grammatical word classes, such as nouns, pronouns, verbs, or articles.

Example -

(1) not — the negative particle
(2) to — the infinitival particle
(3) do, don't, let, let's — the imperative particles

Etymology -
The word derives from the Latin particula, little bit or part.

Oxford English Dictionary -
Its first citation in this sense is from 1533:
"Compowned with theym selfes, they sygnifye as moche as if they were compowned with this partycle cumque, as quisquis, i. quicunque."
(Udall, Flowres, 107)

Example Usages -

1. "In grammar, a particle is a function word that does not belong to any of the inflected grammatical word classes (such as nouns, pronouns, verbs, or articles). It is a catch-all term for a heterogeneous set of words and terms that lack a precise lexical definition."
(Source: Wikipedia, s.v. Grammatical particle )

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