Vocative case
WordNet
noun
(1) The case (in some inflected languages) used when the referent of the noun is being addressed
WiktionaryText
Noun
vocative case
- : case of address, case used for a noun identifying the person or thing being addressed. It corresponds to the English particle "O" as used in solemn or poetic address: Hear me, O Albion! Languages that regularly employ the vocative include Arabic, Croatian, Czech, Greek, Hawaiian, Hindi, Latin, Lithuanian, Ojibwe, Polish, Romanian, Sanskrit, and Serbian.