Here's why:
* Noun: A word that names a person, place, thing, or idea.
* Pronoun: A word that replaces a noun (e.g., he, she, it, they).
* Verb: A word that describes an action or state of being (e.g., run, jump, sleep, be).
Since "noun" is the name of a grammatical category, it fits the definition of a noun.