In linguistics, declension is the inflection of nouns, pronouns, adjectives, and articles to indicate number (at least singular and plural), case (nominative or subjective, genitive or possessive, etc.), and gender. A declension is also a group of nouns that follow a particular pattern of inflection.

Declension occurs in many of the world's languages, and features very prominently in many European languages. Old English was a highly inflected language, as befits its Indo-European and especially its Germanic linguistic ancestry, but its declensions greatly simplified as it evolved into Modern English.

In Modern English, nouns have distinct singular and plural forms; that is, they decline to reflect their grammatical number. (Consider the difference between book and books.) In addition, a few English pronouns have distinct nominative and objective forms; that is, they decline to reflect their relationship to a verb or preposition. (Consider the difference between he (nominative) and him (dative or accusative), as in "He saw it" and "It saw him".) Further, these pronouns and a few others have distinct possessive forms, such as his. By contrast, nouns have no distinct nominative and objective forms, the two being merged into a single plain case. For example, chair does not change form between "the chair is here" (subject) and "I saw the chair" (direct object). Possession is shown by the clitic -'s attached to a possessive noun phrase, rather than by declension of the noun itself.