Outside is an adverb, an adjective, a preposition or a noun.
We use outside as an adverb or an adjective to mean ‘not in a building’:
It was sunny outside, but not very warm. (adverb)
It’s a bit dark at night. We could put an outside light there. (adjective)
Outside can also mean ‘external’, not part of an existing plan or situation:
They’re calling on outside investors for more support.
We use outside or outside of as a preposition to mean ‘not in a particular place, but near it’:
There’s a chair just outside the room opposite.
She works in a software development company just outside of Dublin.
As a preposition, we also use outside or outside of to mean ‘anywhere else apart from a particular place’:
There is a weight limit of 750g for letters outside Europe.
Outside of can also be used with time expressions to mean ‘excluding’ or ‘apart from’:
Outside of the summer months, the hotel rates are lower.
Outside as a noun is used to refer to the exterior of something. It is more informal than exterior:
The outside of the house is not very attractive, but inside it is beautiful. (or, more formal, The exterior of the house …)
