Is off an adjective

It can be, to mean not on: turned off, out of service, not in use (the power is off) or canceled (the wedding is off). Off can also be an adverb or a preposition.

It is an adjective when referring to nouns (off button, off switch, off position).

Also, in the colloquial use "gone off" (mainly British) to mean spoiled (e.g. the tuna salad has gone off).