Vocative case
GRAMMATICAL CASE USED FOR A NOUN THAT IDENTIFIES A PERSON (ANIMAL, OBJECT ETC.) BEING ADDRESSED OR OCCASIONALLY THE DETERMINERS OF THAT NOUN
Vocative; Vocative Case; O (vocative); Vocative comma
In grammar, the vocative case (abbreviated ) is a grammatical case which is used for a noun that identifies a person (animal, object, etc.) being addressed, or occasionally for the noun modifiers (determiners, adjectives, participles, and numerals) of that noun; the comma that should be applied in such a context is referred to as a vocative comma.