vocative case

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• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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