Prime Material Plane
The 'Prime Material Plane' is the plane of existence in which Oberon resides. It is, in effect, a 'Universe' although as it is known that the Prime Material Plane does NOT contain everything that is known the technical definition of a 'Universe' does not apply.
The Prime Material Plane is, effectively 'infinite' in size. It is also just one of an infinite number of Material Planes all of which inhabit the same space but are separate from each other.
The Prime Material Plane is just one of the 'Inner Planes' of existence. Adjacent to the Prime Material Plane are the Shadowfell and the Feywild (separated from the Prime Material Plane by the Shadow Veil and the Green Veil respectively. Surrounding this are the four elemental planes (Air, Earth, Fire and Water) and the demiplanes (Magma, Ooze, Ash, Ice, Dust, Steam).
The Inner planes are often pictured as being discrete and not all touching each other. There's no simple way to produce a two-dimensional representation of what is effectively a four-dimensional relationship. Suffice it to say that each of the inner planes simultaneously borders each other inner plane, including the Prime Material.
