In grammar, the case of a noun or pronoun is an inflectional form that indicates its grammatical function in a phrase, clause, or sentence. For example, a pronoun may play the role of subject ("I kicked the ball"), of object ("John kicked me"), or of possessor ("That ball is mine"). Languages such as Ancient Greek, Latin, and Sanskrit had ways of altering or inflecting nouns to mark roles which are not specially marked in English, such as the ablative case ("John kicked the ball away from the house") and the instrumental case ("John kicked the ball with his foot"). In Ancient Greek those last three words would be rendered tōi podi (τῷ ποδί), with the noun pous (πούς, foot) changing to podi to reflect the fact that John is using his foot as an instrument (any adjective modifying "foot" would also change case to match). As a language evolves, cases can merge (for instance in Ancient Greek genitive and ablative have merged as genitive), a phenomenon formally called syncretism.

Usually a language is said to "have cases" only if nouns change their form (decline) to reflect their case in this way. Other languages perform the same function in different ways. English, for example, uses prepositions like "of" or "with" in front of a noun to indicate functions which in Ancient Greek or Latin would be indicated by changing (declining) the ending of the noun itself.