Definition - An uninflected part of speech that connects words, phrases, and clauses.

Example -

(1) Bob and Dick (coordinating conjunction)
(2) Both truth and beauty (correlative conjunction)
(3) I left because it sucked (subordinating conjunction)

Etymology -
The term derives from the Latin conjunctionem, the past participle of conjugare, to join together.
Note: the Latin grammatical term was coined as a loan translation of the Greek syndesmos, conjunction.

Oxford English Dictionary -
Its first citation in its part-of-speech sense is from 1388:
"A participle … mai be resoluid into a verbe … and a coniunccion copulatif, as thus, dicens, that is, seiynge, mai be resoluid thus, and seith."
(Wyclif Prol., 57)

Quotation -
"Prepositions show the relation of one notion to another. Conjunctions show the relation of one thought to another. Hence conjunctions for the most part join one sentence to another."
(Source: 1876 Mason English Grammar (ed. 21) §287)

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