From my geographical perspective, it basically means the part of the Midwest on the lakes, maybe minus those states which only kiss the lakes (Illinois and Indiana) or two far east (Ohio), as these states have large regions whose climate, culture and history differ quite a bit from the stuff on the lakes. I'd say it always includes Minnesota, Wisconsin Michigan and probably the Dakotas (definitely culturally), geographically. In my mind, though, the Dakotas are much more defined by being in the Great Plains. My definitition is really heavily weighted geographically and politically, so the "center" in definition would really be more Wisconsin than Minnesota, where the river splits the state.
Whatever it is, it's definitely not a hard-set definition.