If you're talking about physical space:
* Exterior: This refers to the outside of something, as opposed to the inside.
* External: This is similar to exterior, but can also be used in a broader sense to refer to things that are outside of a system or entity.
If you're talking about a concept or idea:
* Exterior: This can be used to describe the outward appearance or presentation of something, in contrast to its inner nature.
* External: This can refer to something that originates from outside, or that is not part of a system or entity.
Other possibilities:
* Exterior: This is the most common and direct opposite.
* External: This is a more general opposite that can be used in various contexts.
* Surface: This refers to the outer layer of something, in contrast to its inner layers.
* Exterior: This can be used to refer to the outside of a building or object, in contrast to its interior.
The best opposite will depend on the specific context in which you are using the word "interior."