What is the opposite of an adjective?

An adjective is a word that describes a noun... So the opposite of that would be a word that doesn't describe a noun...

But seriously, perhaps what you are looking for is the adverb, which is a word that describes a verb. Same concept as an adjective, but very different direction.

Actually, an adverb is a word that modifies a verb, an adjective, or another adverb.