Nexus of the Multiverse
The Nexus of the Multiverse refers to the the center of the multiverse, a plane which affects a large number of other worlds.
The world of Dominaria is known to be the Nexus, but since the plane has only been in existence for a finite amount of time and in fact older planes are known (such as Equilor), this was probably not always the case. Due to the changing nature of the multiverse, one day another plane may replace it.