vocative case

noun1

1. •
   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, Bengali, Bulgarian, Czech, Georgian, Greek, Hawaiian, Hindi-Urdu, Irish, Latin, Latvian, Lithuanian, Ojibwe, Polish, Romanian, Russian, Ruthenian/Rusyn, Sanskrit, Scottish Gaelic, Serbo-Croatian, Slovak, Tamil and Ukrainian.