Prepositions (or more generally, adpositions, see below) are a grammatically distinct class of words whose most central members characteristically express spatial relations (such as the English words in, under, toward) or serve to mark various syntactic functions and semantic roles (such as the English words of, for). In that the primary function is relational, a preposition typically combines with another constituent (called its complement) to form a prepositional phrase, relating the complement to the context in which the phrase occurs.

The word preposition comes from Latin, a language in which such a word is usually placed before its complement. (Thus it is pre-positioned.) English is another such language. In many languages (e.g. Urdu, Turkish, Hindi and Japanese), the words with this grammatical function come after, not before, the complement. Such words are then commonly called postpositions. Similarly, circumpositions consist of two parts that appear on both sides of the complement. The technical term used to refer collectively to prepositions, postpositions, and circumpositions is adposition. Some linguists use the word "preposition" instead of "adposition" for all three cases.