I'm placing this post up because I just had an interview with Apple and I couldn't for the life of me remember what the definition to a interface was. I feel dumb because I talked to my roommate and described it perfectly. I hate it when I'm nervous.
An interface in Objective-C is like in the .h file. You have the interface that describes what the class looks like and provides the interaction with the class. Essentially, it is the interaction layer between two classes. Of course, when you think of it on a different scale, an interface could be anything that provides an interaction. Like the UI, it is an interface. It provides interaction. API (Application Programming Interface), It's another interface.
I have to remember this so if I get another interview I won't embarrass myself!