The Overworld is a term used to describe the normal world the player spawns in. Usually when it is used, it is to distinguish it from the Nether or the End, but in the Hexxit modpack it is also used to distinguish the familiar minecraft world from the Twilight Forest, Limbo, and Pocket Dimensions (the Twilight Forest is from the eponymous mod, and Limbo and Pocket Dimensions are from the Dimensional Doors mod).

An example of a small corner of the Overworld.