If you're asking about part of speech, it's just a noun. But it sounds more like you're asking about its role in the sentence (like subject or object or whatever), which is indeed a vocative. A vocative is always some type of noun phrase, whether it's a common noun, a name, or a pronoun, and it's used to address or invoke a person or thing.
Some more examples:
Nice job on that car stereo, sweet clementine!
Hey, you, do you know what time it is?
Hi, Mom.
Guys, where do you want to go for dinner?