Definition - The set of forms that inflected languages use to show word relationships in a sentence.

1. The set of cases for the class to which a word belongs is called a paradigm.

Example -

(1) In Latin the noun form dominus (lord) is in the nominative case, which is used when the word is the sentence's subject.
(2) Dominum is the noun form used in the accusative case, which is used when the word is a direct object.

Etymology -
The word derives via Old French from the Latin casus, a falling (from cadere, to fall).

Oxford English Dictionary -
Its first citation in its grammatical sense is from 1393:
"[As] adiectif and substantyf vnite asken Acordaunce in kynde in cas and in numbre."
(Langl. P. Pl. C. iv. 339)

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