Appositives are words, phrases, and clauses that support another word, phrase, or clause by describing or modifying the other word, phrase, or clause.
Appositive is a grammatical function.
The grammatical forms that can function as the appositive in English grammar are the noun phrase, noun clause, adjective phrase, verb phrase, and adverb phrase.
Hopper, Paul J. 1999. A short course in grammar. New York: W. W. Norton & Company.