Multiverse (DC Comics)
In DC Comics, the Multiverse was a continuity construct in which multiple fictional versions of the universe existed in the same space, separated from each other by their vibrational resonances. Each universe varied from the others, in either subtle or profound ways. In particular, the Earth of each universe had a different set of superheroes, or the life histories of its superheroes were different from those of others'. In several cases, the characters from publishers acquired by DC were assigned their own universe. The universes were identified within the comics in reference to these Earths, known as "Earth-One", "Earth-Two", "Earth-Three", "Earth-X", and so forth. These multiple Earths were eliminated in Crisis on Infinite Earths, a mini-series published in 1985.
Earth-One
Home of DC's Silver Age heroes, including the original Justice League of America. In this world, Superman had a career as Superboy while in his teens, and an array of other heroes first appeared in the 1950s or later. The Flash was police scientist Barry Allen, Green Lantern was galactic peacekeeper Hal Jordan, Hawkman was Thanagarian Katar Hol, and the Atom was shrinking scientist Ray Palmer.
Earth-Two
Home of DC's Golden Age heroes, including the original Justice Society of America. In this world, Superman, Batman, and Wonder Woman began their careers in the late 1930s (concurrent with their first appearance in comics). The Flash was chemistry student Jay Garrick, Green Lantern was the magic-powered Alan Scott, Hawkman was archaeologist Carter Hall, and the Atom was the short powerhouse Al Pratt. Characters unique to this world included the Star-Spangled Kid, Huntress, The Spectre, Johnny Thunder, Dr. Fate, and Mr. Terrific. The heroes of this world appeared as comic book characters on Earth-One.
Earth-Three
Home of evil versions of Earth-One heroes, including the Crime Syndicate of America. History was "backwards" here. For example, Christopher Columbus was an American who discovered Europe; Britain fought and won its freedom from America in the Revolutionary War; President John Wilkes Booth was assassinated by actor Abraham Lincoln; the United States flag had 'reversed' colors (black stars on a red background, and alternating blue and black stripes); and so on. Noteworthy characters include Ultraman (Superman), Superwoman (Wonder Woman), Owlman (Batman), Power Ring (Green Lantern), Johnny Quick (Flash), and Alexander Luthor (an heroic version of Lex Luthor).
Earth-Four
Home of the former Charlton Comics heroes. Noteworthy characters include Captain Atom, Blue Beetle, Nightshade, Peacemaker, The Question, Peter Cannon, Thunderbolt, and Judomaster.
Earth-C-Minus
Home of funny animal superheroes, including Justa Lotta Animals.
Earth-Q
Home of the former Quality Comics heroes as they were originally published by Quality in the 1940s (i.e. not the later versions published by DC, on Earth-X).
Earth-S
Home of the former Fawcett Comics heroes, including the Captain Marvel (Shazam!) family. Other noteworthy characters include Bulletman, Bulletgirl, Ibis the Invincible, and Spy Smasher.
Earth-X
Home of the former Quality Comics heroes, including the Freedom Fighters. On this world, Nazi Germany won World War Two, and the Freedom Fighters fought to defeat them. Noteworthy characters include Uncle Sam, the Human Bomb, Miss America, The Ray, Black Condor, Doll Man, Phantom Lady, Firebrand, and the Red Bee.
Earth-Prime
Home of few or no superheroes; this was supposedly "our" world, where the characters of Earth-One were merely comic book characters. (The continued existence of the real world following the destruction of Earth-Prime in Crisis suggests that it was not, in fact, our world.) DC editor Julius Schwartz lived here, and met the Flash of Earth-One who visited this world. The little-used character Ultraa was described as the only superhero of this world. In a story just before Crisis on Infinite Earths, a young Clark Kent discovered that he had super powers and became Superboy; at the climax of the Crisis, he joined the Superman and Lois Lane of Earth-Two (the originals) in a kind of paradise.