Refers to the DC Comics continuity as it existed before the events described in the mini-series Crisis On Infinite Earths.

Pre-Crisis, the DC universe was a multiplicity of alternate worlds with names like Earth-1, Earth-Two, Earth-3, Earth-S, and Earth-X. The writers who dreamed up these parallel Earths originally did so to explain the differences between the heroes as presented in the 1940s and those same heroes as they appeared in the 1960s--the answer given was they inhabited alternate realities.

Once that idea was turned loose, the writers ran with it like crazy. The multiverse was used to provide a source for new villains (Earth-3) or to incorporate heroes created by other companies, the rights to which had been purchased by DC (Earths S and X).

During the Crisis story, all the worlds were merged into one continuity, though this hasn't stopped some writers from bending the rules a bit now and then.