Greyspace

Greyspace was the crystal sphere that contained the planet Oerth and the other planets of its system. Unlike Realmspace, which had the sun as the sphere's primary celestial body, Oerth was Greyspace's primary, sitting at the center of the sphere and orbited by its sun and other planets.

Geography
Greyspace was part of a triangular configuration in the flow of the phlogiston that also included Realmspace and Krynnspace. The flow allowed two-way spelljammer travel between Realmspace and Greyspace, as well as one-way travel from Greyspace to Krynnspace.