Declension

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 (also called subjective) and oblique (or objective) forms; that is, they decline to reflect their relationship to a verb or preposition, or case. Consider the difference between he (subjective) and him (objective), as in "He saw it" and "It saw him"; similarly, consider who, which is subjective, and the objective whom. Further, these pronouns and a few others have distinct possessive forms, such as his and whose. 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.

...
Wikipedia