In grammar, a conjunction is a part of speech that connects two words, phrases, or clauses together. This definition may overlap with that of other parts of speech, so what constitutes a "conjunction" should be defined for each language. In general, a conjunction is an invariable grammatical particle, and it may or may not stand between the items it conjoins.

The definition can also be extended to idiomatic phrases that behave as a unit with the same function as a single-word conjunction (''as well as'', ''provided that'', etc.).

Types of conjunctions

Coordinating conjunctions, also called coordinators, are conjunctions that join two items of equal syntactic importance. As an example, the traditional view holds that the English coordinating conjunctions are ''for'', ''and'', ''nor'', ''but'', ''or'', ''yet'', and ''so'' (which form the mnemonic ''FANBOYS''). Note that there are good reasons to argue that only ''and'', ''but'', and ''or'' are prototypical coordinators, while ''nor'' is very close. ''So'' and ''yet'' share more properties with conjunctive adverbs (e.g., ''however''), and "''for''...lack(s) most of the properties distinguishing prototypical coordinators from prepositions with clausal complements" p. 1321, Huddleston, R. Payne, J. & Peterson, P. (2002). Coordination and supplementation. Pp. 1273-1362. In Huddleston, R. & Pullum, G. K. ''The Cambridge Grammar of the English Language''. Cambridge: Cambridge University Press.. Furthermore, there are other ways to coordinate independent clauses in English.

Correlative conjunctions are pairs of conjunctions which work together to coordinate two items. English examples include ''both … and'', ''either … or'', ''neither … nor'', and ''not (only) … but (… also)''.

Subordinating conjunctions, also called subordinators, are conjunctions that introduce a dependent clause. English examples include ''after'', ''although'', ''if'', ''unless'', and ''because''. Complementizers can be considered to be special subordinating conjunctions that introduce complement clauses (e.g., "I wonder ''whether'' he'll be late. I hope ''that'' he'll be on time"). Some subordinating conjunctions (''although'', ''before'', ''until'', ''while''), when used to introduce a phrase instead of a full clause, become prepositions with identical meanings.

In many verb-final languages, subordinate clauses ''must precede'' the main clause on which they depend. The equivalents to the subordinating conjunctions of non-verb-final languages like English are either

''clause-final conjunctions'' (e.g. in Japanese) or;

''suffixes'' attached to the verb and ''not'' separate wordsDryer, Matthew S. 2005. "Order of adverbial subordinator and clause". In The World Atlas of Language Structures, edited by Martin Haspelmath, Matthew S. Dryer, David Gil, and Bernard Comrie. Oxford University Press. ISBN 0199255911