Multiverse

The Multiverse is the totality of all of creation, including the Prime Material Plane, the Outer Planes, the Ethereal and Astral planes and other transitive planes, as well as a wide range of demi-planes and other cosmological odds and ends.

Some people think that the Multiverse also includes multiple sets - perhaps even an infinite number - of copies of that wide range of planes and locations, each one slightly different than the other. This is the broadest application of the concept of multiple worlds.