Wizards on the material plane only have access to a limit number of alternate universes - those directly surrounding ours. Great Wizards and Elf Kings have access to the heavenly spheres; lesser magical beings may step between the other earths.
higher, purer universes than ours - places that govern hidden rules and truths of our universe
Aether - the fabric of the universe - the fifth element - starmatter - divine truth
Qi - the flow of energy within all things - what allows a man to be a man, and not a tree - inner truth
Limbo - nothingness. a meeting ground for great beings.
Entropy - pure chaos - pure reality - maddening but powerful to behold
Dreaming - the collective unconsciousness; the realm of collected ideas and imaginations
the Other Earths
alternate incarnations of the prime material plane. for some reason, they're all either dead or dying.
Ash World - a supervolcano has blocked out the sun. the world is a desert of ash.
Flooded World - the seas rose, and the earth crumbled. 95% of the earth is covered in water.
the Wasteland - the world is completely identical to the prime material plane, but devoid of non-plant life
Forgotten World - humans never evolved, neanderthals took their place in history. lots of dinosaurs.
War World - entire nations fight one another in giant sophisticated mobile war citadels over resources.
the Negaverse - the mirror universe - good is evil and evil is good.