Pronoun

I belong to one of the major form classes in any of a great many languages. I am a substitute for a noun or noun equivalent, take noun constructions, and am declined. I refer to persons or things named, asked for, or understood in context. I have little or no fixed meaning except one of relation or limitation. I take many forms: emphatic, identifying, intensive, personal, reciprocal, refexive, and relative.

I am, in fact, not a noun, but its substitute.