Is smooth an adverb or an adjective?

Answer 1

The word 'smooth' is not a noun.

The word 'smooth' is a verb (smooth, smooths, smoothing, smoothed), and an adjective (smooth, smoother, smoothest).

The noun form of the verb to smooth is the gerund, smoothing.

The noun form of the adjective smooth is smoothness.

Answer 2

"Smooth" is usually an adjective, but can be an adverb when it means "smoothly, in a smooth fashion."

e.g. the motor runs smooth, the wood must be sanded smooth.

(Purists might contend that the motor runs smooth is incorrect, it should be smoothly, but this is probably one of those cases where we have to accept that the language is changing.)

It can also be a verb.

Answer 3

Yes, smooth is an adjective.