Earth-One was the designation for an Earth, and the Universe it inhabited, that existed prior to the Crisis on Infinite Earths. Earth-One was the default Earth for most of DC's comics during the time the original Multiverse construct was in use (The time period is approximately 1956 to 1970), Earth-One was also by far the most populated and widely explored. First described as a distinct Earth in Flash (vol. 1) #123 (September 1961), first named in Justice League of America (vol. 1) #21 (August 1963)
Crisis on Infinite Earths (1985–1986) was an effort by DC Comics to clean up their continuity, resulting in the multiple universes, including that of Earth-One, combining into one. This involved the destruction of the multiverse, including Earth-One and the first appearance of the post-Crisis Earth.