Prepositions and postpositions, together called adpositions (or broadly, in traditional grammar, simply prepositions), are a class of words used to express spatial or temporal relations (in, under, towards, before) or mark various semantic roles (of, for).

A preposition or postposition typically combines with a noun phrase, this being called its complement, or sometimes object. A preposition comes before its complement; a postposition comes after its complement. English generally has prepositions rather than postpositions – words such as in, under and of precede their objects, such as in England, under the table, of Jane – although there are a few exceptions including "ago" and "notwithstanding", as in "three days ago" and "financial limitations notwithstanding". Some languages that use a different word order have postpositions instead, or have both types. The phrase formed by a preposition or postposition together with its complement is called a prepositional phrase (or postpositional phrase, adpositional phrase, etc.) – such phrases usually play an adverbial role in a sentence.

A less common type of adposition is the circumposition, which consists of two parts that appear on each side of the complement. Other terms sometimes used for particular types of adposition include ambiposition, inposition and interposition. Some linguists use the word preposition in place of adposition regardless of the applicable word order.