vocative case

Wiktionary

1. n. grammar 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: Hear me, O Albion! Languages that regularly employ the vocative include Arabic, Czech, Georgian, Greek, Hawaiian, Hindi, Irish, Latin, Lithuanian, Ojibwe, Polish, Romanian, Sanskrit, Serbo-Croatian, and Ukrainian.

WordNet 3.0

1. n. the case (in some inflected languages) used when the referent of the noun is being addressed