From superman.nu: Supermanica
One of the countless Parallel-Worlds that Superman has encountered in his adventures. This version of Earth is portrayed as a direct continuation of the Golden Age (1935-1947) of the chronicles, although it differs in several details from the original Golden Age stories. Its greatest distinction from the Earth of Superman (Earth-1) is that Earth-1 is the location of the modern-day adventures of the successors of Superman and his contemporaries (Batman, Wonder Woman, etc.) since, on Earth-2, Superman's greatest period of activity was during the Golden Age, not in the present day. Earth-2 was first encountered by the Flash (The Flash No. 123, Sep 1961: "Flash of Two Worlds!").