Extraplanar Realm

"Perhaps the greatest-kept secret of the mages is not the knowledge that there are countless worlds beyond our own, but that that ours in not the only reality."- Archmage Samuel Firgold
An Extraplanar Realm is a set of planes of existence or singular plane of existence that cosmically overlaps the Prime Material plane but does not exist inside of an alternate reality. Most realities contain an equal set of Extraplanar Realms, and each of these are as varied and unique as the realities they spawn from. Time does not pass in a logical or predictable fashion between most Extraplanar Realms in relation to their home reality. The key identified difference between an Extraplanar Realm and a Plane of Existence within the same reality is the way in which magic works and affects the world. For instance, a spell cast by a wizard from the Material Plane works the same way on the Plane of Fire or Hell, but would work drastically different if at all in an Extraplanar Realm such as Planet Earth or the Toon Realm. Some Extraplanar Realms simultaneously exist within multiple alternate realities or travel freely between them, and some are outside of the known multiverse as it is understood. 
Every possible existence, be it planar, extraplanar, or alternate reality all together is known as the Omniverse, the term Multiverse applies only to the distinction between realities. Reality and Timeline mean functionally the same thing, though timeline is more frequently used to discuss very similar or adjacent realities, where the only differences stem from a single choice. Differences of Extraplanar Realms are tied heavily to their home reality. 
A reality's Core Realm is the material plane from which new realities stem. In most known realities, this is the Prime Material plane tied in some way to the Empire of Evermore.