The word is 'case'.
As the only form of the noun recognisable by a 'case-ending' is the genitive (possessive) 's, we do not generally use the word 'case' in talking about English nouns.
We do have 'nominative' and 'accusative' forms of personal pronouns, such as I/me, they/them,etc, but we generally prefer the terms 'subject' and 'object' pronouns.
Unless they have learnt a language such as Latin or German at school, most English speakers do not understand such words as case (in this sense), declension. decline, nominative, vocative, accusative, genitive, dative, ablative, locative.
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