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