DC Multiverse

The Multiverse, within DC Comics publications, is a "cosmic construct" collecting many of the fictional universes in which the published stories take place. The worlds in this multiverse share a space and fate in common and its structure has changed several times in the history of DC Comics.

The concept of a universe and a multiverse in which the fictional stories take place was loosely established during the Golden Age. With the publication of All-Star Comics #3 in 1940, the first crossover between characters occurred with the creation of the Justice Society of America (JSA) which presented the first super hero team with characters appearing in other publications (comic strips and anthology titles) to bring attention to less known characters. This established the first shared "universe", as all these heroes now lived in the same world. Prior to this publication, characters from the different comic books seemingly existed in different worlds.

Later, Wonder Woman #59 (May 1953) presented DC Comics' first story depicting a parallel "mirror" world. Wonder Woman is transported to a twin Earth where she meets Tara Terruna who is exactly like her. Tara Terruna means "Wonder Woman" in the native language of that world. Wonder Woman describes this world as being a twin world existing alongside Earth with duplicates of everyone but with a different development. The concept of different versions of the world and its heroes was revisited in the pages of Wonder Woman a few times later.

Led by editor Julius Schwartz and writer Gardner Fox, DC Comics' super heroes were given a "reboot" with the publication of Showcase #4 in 1956 where a new version of The Flash made its first appearance. The success of this new Flash led the creation of new incarnations of the Golden Age characters who only shared the names and powers but had different secret identities, origins and stories. Later, new versions of other heroes, namely, Superman, Batman and Wonder Woman, were also restarted by retelling their origins but keeping their secret identities. Gardner Fox, who worked before in the creation of the JSA, where other heroes met for the first time, created the story "Flash of Two Worlds" in The Flash #123, where Barry Allen, the new Flash, is transported to the Earth where the original Flash, Jay Garrick, existed. To Allen, Jay Garrick's world was a work of fiction as it was in the real world. This story not only presented the encounter of two worlds and the existence of the Multiverse for the first time, it also presented key features of the Multiverse: All the universes vibrate at a specific frequency which keeps them separated; these "barriers" could by trespassed by "tuning" to that vibration. Because people could also "tune-in" these worlds in dreams, some people wrote comic books with the stories from those worlds they dreamed, which explained why Barry Allen knew about Jay Garrick as a fictional character.

...
Wikipedia