Earth-One (also Earth-1) is a name given to two fictional universes (the Pre-Crisis and Post-Crisis versions of the same universe) that have appeared in American comic book stories published by DC Comics. The first Earth-One was given its name in Justice League of America #21 (August 1963), after The Flash #123 (September 1961) explained how Golden Age (Earth-Two) versions of characters such as the Flash (Jay Garrick) could appear in stories with their Silver Age counterparts (Barry Allen). This Earth-One continuity included the DC Silver Age heroes, including the Justice League of America.
Earth-One, along with the four other surviving Earths (Earth-Two, Earth-Four, Earth-S, and Earth-X) of the DC Multiverse, are merged into one in the 1985 miniseries Crisis on Infinite Earths. This Earth's versions of characters were primarily the Earth-One versions (i.e. Superman, Batman), but some characters from the four other worlds were also "folded" in. In Infinite Crisis, Earth-One was resurrected and merged with the primary Earth of the publication era to create a New Earth that brought back more aspects of Earth-One's original history. In 2007, a new version of Earth-One was created in the aftermath of events that occurred within the 52 series.
Pre-Crisis version
Flash of Two Worlds
Characters from DC Comics were originally suggestive of each existing in their own world, as superheroes never encountered each other. This was soon changed with alliances being formed between certain protagonists. Several publications, including All-Star Comics (publishing tales of the Justice Society of America), Leading Comics (publishing tales of the Seven Soldiers of Victory) and other comic books introduced a "shared-universe" among several characters during the 1940s until the present day.
Alternative reality Earths had been used in DC stories before, but were usually not referred to after that particular story. Also most of these alternative Earths were usually so vastly different that no one would confuse that Earth and its history with the so-called real Earth. That would change when the existence of another reliable Earth was established in a story titled "Flash of Two Worlds" in which Barry Allen, the modern Flash later referred to as Earth-One (the setting of the Silver Age stories) first travels to another Earth, accidentally vibrating at just the right speed to appear on Earth-Two, where he meets Jay Garrick, his Earth-Two counterpart.