adjective
Definition: Relating to a grammatical case used to indicate the person or thing being directly addressed in speech or writing.
Example: In the sentence 'O Lord, hear my prayer', 'O Lord' is in the vocative case.
noun
Definition: A word or form used to address someone directly, often found in the vocative case in grammar.
Example: In the sentence 'Hello, John!', 'John' is in the vocative case.