Compatibility can be achieved in several ways. Read this: http://en.wikipedia.org/wiki/Backward_compatibility. Perhaps compatibility with regards to different aspects (GC discs, GC hardware, GBA software/hardware) should be mentioned in the article. To what extent an emulator can be a called a method of providing compatibility for a given platform I don't know. Neither forementioned page nor this one http://en.wikipedia.org/wiki/Emulator, clearly gives the boundaries of the definition.