The Multiverse

The Multiverse of Magic: the Gathering is the near-infinite number of different planes.

Description
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Dominia is an outdated name for the entire Multiverse.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 have been the Nexus. Since the plane had 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, it was bound to happen that one day another plane would replace it. Dominaria stopped being the Nexus of the Multiverse when the Mending happened. It is currently unknown where the new nexus is located.

Being a Nexus is probably connected with a large amount of mana lines flowing into it from other planes. That lets it 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.

Un-iverse
The silver-bordered Multiverse is known as the Un-iverse. This is a separate entity , a parallel universe.

Characters, in slightly different forms, can exist in both Multiverses at the same time.

Planar Chaos
The multiverse interacted with an alternate reality, maybe a parallel multiverse, in the Planar Chaos storyline.

Earth
Earth is not considered to be part of either Multiverse. This is because R&D doesn't want to tell stories about planeswalkers visiting Earth or being from Earth.

The setting of Portal Three Kingdoms closely resembles historic Earth, but is retroactively considered to be a parallel plane.

Database
Multiverse also used to be the name of the Wizards of the Coast card database that includes R&D comments on cards. It was replaced by a new database named Drake.