In morphology and syntax, a clitic (, backformed from Greek ἐγκλιτικός enklitikós "leaning" or "enclitic") is a morpheme that has syntactic characteristics of a word, but depends phonologically on another word or phrase. In this sense, it is syntactically independent but phonologically dependent—always attached to a host. A clitic is pronounced like an affix, but plays a syntactic role at the phrase level. In other words, clitics have the form of affixes, but the distribution of function words.

Clitics can belong to any grammatical category, although they are commonly pronouns, determiners, or adpositions. Note that orthography is not always a good guide for distinguishing clitics from affixes: clitics may be written as separate words, but sometimes they are joined to the word they depend on (like the Latin clitic -que, meaning "and") or separated by special characters such as hyphens or apostrophes (like the English clitic 's in "it's" for "it has" or "it is").