Vocative case

The vocative case (abbreviated , voc.) is the case used for a noun that identifies a person (animal, object, etc.) being addressed or, occasionally, the determiners of that noun. A vocative expression is an expression of direct address where the identity of the party spoken to is set forth expressly within a sentence. For example, in the sentence, "I don't know, John", John is a vocative expression that indicates the party being addressed—as opposed to the sentence, "I don't know John", where John is the direct object of the verb "know."