Is outside a preposition or an adverb?

The term "outside" can be used as a preposition AND an adverb - depending on how the sentence is phrased.

For example:

I walked outside. (In this sentence, "outside" is an adverb because "I walked" can stand alone as an individual sentence.)

Outside of the house, the winds roared. (In this case, "outside" is a preposition because the rest of the sentence cannot stand alone.)