The multiverse

The Multiverse is the near-infinite number of different planes. Nobody really knows how many planes there truly are. Urza for one believed that there is a finite number of planes, merely more than can ever be counted in the combined lifetime of every being in existence. However another source notes that the multiverse is "boundless".

Dominia
Dominia is an outdated name for the entire multiverse. The term was dropped probably due to its confusing similarity to Dominaria, the setting for most of Magic's storylines in the early years.

Nexus of The Multiverse
The Nexus of the Multiverse is a plane (or a place) in the center of the multiverse with a great influence on all other planes. 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.

Being a Nexus is probably connected with the large amount of mana lines flowing into it from other planes. That lets it to quickly regenerate its supplies of mana and most likely is a way of how it affects other planes. Its collapse can cause the destruction of the time stream — an abrupt end of the multiverse's existence and the near-infinite amount of its planes.