In grammar, a preposition is a part of speech that introduces a prepositional phrase. For example, in the sentence "The cat sleeps on the sofa", the word "on" is a preposition, introducing the prepositional phrase "on the sofa". In English, the most used prepositions are "of", "to", "in", "for", "with" and "on". Simply put, a preposition indicates a relation between things mentioned in a sentence.

Linguists sometimes distinguish between a preposition, which precedes its phrase, a postposition, which follows its phrase, and as a rare case a circumposition, which surrounds its phrase. Taken together, these three parts of speech are called adpositions. In more technical language, an adposition is an element that, prototypically, combines syntactically with a phrase and indicates how that phrase should be interpreted in the surrounding context. Some linguists use the word "preposition" instead of "adposition" for all three cases.