A conjunction is part of speech/  word that connects other words or groups of words that serves to connect words, phrases, clauses, or sentences.
This definition may overlap with that of other parts of speech, so what constitutes a "conjunction" must 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 may also be extended to idiomatic phrases that behave as a unit with the same single-word conjunction (as well as, provided that, etc.).
The common conjunctions--and, but, for, or, nor, yet, and so--join the elements of a coordinate structure.
A sentence style that employs many coordinate conjunctions is called polysyndeton. A sentence style that omits conjunctions between words, phrases, or clauses is called asyndeton.
In contrast to coordinating conjunctions, which connect words, phrases, and clauses of equal rank, subordinating conjunctions connect clauses of unequal rank.