介词、介系词（英语：preposition）在语法里是一个用来表现一个字的文法功能的词汇或字缀。

介词是虚词的一种，本身没有实在的意义，主要用在名词、代词前面，组成介词结构，表示下面几种意思︰

Prepositions and postpositions, together called adpositions (or broadly, in English, simply prepositions), are a class of words that express spatial or temporal relations (in, under, towards, before) or marking various semantic roles (of, for).

A preposition or postposition typically combines with a noun or pronoun, or more generally 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 small handful of exceptions including "ago" and "notwithstanding", as in "three days ago" and "financial limitations notwithstanding". Some languages, which 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.