In linguistics, declension (verb: to decline) is the changing of the form of a word, generally to express its syntactic function in the sentence, by way of some inflection. Declensions may apply to nouns, pronouns, adjectives, adverbs, and articles to indicate number (e.g. singular, dual, plural), case (e.g. nominative case, accusative case, genitive case, dative case), gender (e.g. masculine, neuter, feminine), and a number of other grammatical categories. Meanwhile, the inflectional change of verbs is called conjugation.

Declension occurs in many of the world's languages. It is an important aspect of language families like Quechuan (i.e., languages native to the Andes), Indo-European (e.g. German, Lithuanian, Latvian, Slavic, Sanskrit, Latin, Ancient Greek, Modern Greek, Albanian, Classical Armenian and Modern Armenian and Kurdish), Bantu (e.g. Zulu, Kikuyu), Semitic (e.g. Modern Standard Arabic), Finno-Ugric (e.g. Hungarian, Finnish, Estonian), and Turkic (e.g. Turkish).

Old English was an inflectional language, but largely abandoned inflectional changes as it evolved into Modern English. Though traditionally classified as synthetic, Modern English has moved towards an analytic language.