"Many similar universes co-exist in the same space, but in an infinite number of different dimensions! There's no limit to the variations..." - Superboy (SB No.117, Dec 1964)
The sum total of all universes referred to collectively.
Numerous parallel worlds and universes have been documented. These universes are usually designated by the version of the earth that exists within them. For example, the universe that is home to the villainous analogue of Superman, Ultraman, is designated "Earth-3" and an alternate universe created by the villainous Johnny Thunder of Earth-1 (in which Superman and other members of the Justice League of America do not exist) is known as "Earth-A".
There are at least 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.
There are also at least four similar versions of Superboy, one from a world designated Earth-1, another from a world designated as Earth-Prime, and still two others from worlds that were never designated. These four boys, while very similar, also have different histories and are clearly different people.
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, approximately 1938-1955, are actually those of the Superman of Earth-2.