It is the vocative. The vocative expresses the person, people, animal, or thing being addressed with the sentence. The word vocative refers to calling (invoke, c.f. vocal, revoke, convocation, etc.).
God, give me strength. (vocative) vs. God give me strength. (subject with subjunctive verb.)
Fido, come here.
Susan, I just don't know.
Doctor, what is the matter?
Death, where is thy victory?
Friends, Romans, countrymen, lend me your ears.