Vocative Case

From Wikipedia, "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 by which 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" in which "John" is the direct object of the verb "know."