The Multiverse

The sum total of all universes referred to collectively.

Numerous parallel-worlds and universes have been documented. These universes are usually designated by the the version of the earth that exist in them. So the universe which is home to the villainous analogue of Superman, Ultraman of Earth-3, is designated Earth-3 and the alternate universe created by a villainous parallel version of Johnny Thunder in which Superman and the other members of the Justice League of America do not exist is known as Earth-A.

There are two similar versions of Superman, one from a world designated Earth-1 and the other from a world designated Earth-2. These two men while very similar have different histories and are clearly different people as demonstrated by the fact that they have met on occasion.

The man most people know as Superman is in fact the Superman of Earth-1 though it is claimed by some Superman historians that the earliest chronicles of the Man of Steel are actually those of the Superman of Earth-2.