In grammar, a conjunction (abbreviated CONJ or CNJ) is a part of speech that connects words, phrases, or clauses that are called the conjuncts of the conjunctions. That definition may overlap with that of other parts of speech, and so what constitutes a "conjunction" must be defined for each language. In English, a given word may have several senses, and be either a preposition or a conjunction depending on the syntax of the sentence. For example, after is a preposition in "he left after the fight" but is a conjunction in "he left after they fought". In general, a conjunction is an invariable (non-inflected) grammatical particle that may or may not stand between the items conjoined.

The definition of conjunction may also be extended to idiomatic phrases that behave as a unit with the same function, e.g. "as well as", "provided that".

A simple literary example of a conjunction is "the truth of nature, and the power of giving interest" (Samuel Taylor Coleridge's Biographia Literaria).

A conjunction may be placed at the beginning of a sentence to c: "But some superstition about the practice persists."