The Multiverse
Dominia was the general name of the set of infinite planes that make up the Multiverse where the stories of the Magic: The Gathering occur. According to an "Ask Wizards" article recently, the term 'Dominia' is no longer being used to define the Magic: The Gathering Multiverse.
Each plane is a universe. However, many gameplayers confuse the term 'plane' to a planet when, in fact, it is the entire universe where that certain planet is located. This is because the name of the universe/plane is similar to the name of the main/primary planet.
However, some planes aren't physically universes (per se) but, merely, single planets floating in the void. These planes aren't natural but artificial (i.e. Phyrexia, Serra's Realm, Rath, and Mirrodin). More often than not, artificial planes are planes that comprise the main/primary planet itself, a few satellites and little else (unlike natural planes which have galaxies and other planets). Artificial planes have limited lifespans. It may take thousands of years, but each will eventually break down.
Planeswalkers have a near-monopoly on travelling plane-to-plane. However, planeswalkers aren't invulnerable to events of cosmic-scale, such as the cosmic anomaly known as the The Shard of Twelve Worlds. Planeswalkers who were, at that time, venturing within the realms of the Twelve Worlds of the Shard (also known as Nexus of Dominaria) were trapped within the Shard for thousands of years.
Read more about this topic: Plane (Magic: The Gathering)