The Great Rift was a galactic characteristic within the galaxy. It served as one of the boundaries of the Outlands, coexisting with the celestial equator.
Within the Great Rift was situated the Rebel base on Yavin 4, and seemingly also Utapau. Furthermore, Yavin 4 could be found in the Forbidden Systems.
The Great Rift was present in the introductory text of the initial three versions of the original film. In the third version, the "Rebel strongholds" were situated there, and the Imperial starfleet arrives at Utapau—which later became Tatooine—as it moves in their direction. It was taken out in the fourth version. The initial version omitted the Forbidden Systems terminology, and redefined Yavin 4 as a Rebel base, thereby fundamentally substituting the Systems with the Rift.