Particle

From Middle French particule, and its source, Latin particula ("small part, particle"), diminutive of pars ("part, piece").

Full definition of particle

(plural particles)

A very small piece of matter, a fragment; especially, the smallest possible part of something. from 14th c.
(linguistics, sensu lato) A part of speech which cannot be inflected: an adverb, preposition, conjunction or interjection.

1844, E. A. Andrews: First Lessions in Latin; or Introduction to Andrews and Stoddard's Latin Grammar. (6th edition, Boston), p.91 (at books.google)322. The parts of speech which are neither declined nor conjugated, are called by the general name of particles. 323. They are adverbs, prepositions, conjunctions, and interjections.
1894 (2008), B. L. Gildersleeve & G. Lodge: Gildersleeve's Latin Grammar (reprint of the 3rd edition by Dover, 2008), p.9. (at books.google)The Parts of Speech are the Noun (Substantive and Adjective), the Pronoun, the Verb, and the Particles (Adverb, Preposition, and Conjunction).

(linguistics, sensu stricto) 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 the vocative particle.

1965-06-04, Shigeyuki Kuroda, Generative grammatical studies in the Japanese languageIn English there is no grammatical device to differentiate predicational judgments from nonpredicational descriptions. This distinction does cast a shadow on the grammatical sphere to some extent, but recognition of it must generally be made in semantic terms. It is maintained here that in Japanese, on the other hand, the distinction is grammatically realized through the use of the two particles wa and

ga.

1988, Andrew Radford, Transformational grammar: a first courseTraditional grammar typically recognises a number of further categories: for example, in his Reference Book of Terms in Traditional Grammar for Language Students, Simpson (1982) posits two additional word-level categories which he refers to as Particle, and Conjunction. Particles include the italicised words in (58) below:; (58)
(a) He put his hat on(b) If you pull too hard, the handle will come off(c) He was leaning too far over the side, and fell out(d) He went up to see the manager

(physics) Any of various physical objects making up the constituent parts of an atom; an elementary particle or subatomic particle. from 19th c.

2011, Brian Cox & Jeff Forshaw, The Quantum Universe, Allen Lane 2011, p. 55:What, he asked himself, does quantum theory have to say about the familiar properties of particles such as position?
2012, Jeremy Bernstein, A Palette of Particles, The physics of elementary particles in the 20th century was distinguished by the observation of particles whose existence had been predicted by theorists sometimes decades earlier.