The case (in some inflected languages) used when the referent of the noun is being addressed WordNet 3.0 & Open English WordNet

In grammar, the vocative case is a grammatical case which is used for a noun that identifies a person Wikipedia

Grammatical case used for a noun that identifies a person being addressed or occasionally the determiners of that noun Wikidata

The grammatical case indicating the person or thing being called upon or addressed in a sentences of invocation, begging, ordering, wish or asking. OmegaWiki

A word in the vocative case. Wiktionary

Case of address, case used for a noun identifying the person or thing being addressed. It corresponds to the archaic English particle "O" as used in solemn or poetic address e.g.: Hear me, O Albion! Languages that regularly employ the vocative include Arabic, Bulgarian, Czech, Georgian, Greek, Hawaiian, Hindi, Irish, Latin, Latvian, Lithuanian, Ojibwe, Polish, Romanian, Ruthenian/Rusyn, Sanskrit, Scottish Gaelic, Serbo-Croatian, Slovak, Slovenian, Tamil and Ukrainian. Wiktionary

Case of address. Wiktionary (translation)