Multiverse (DC Comics)
The DC Multiverse is a fictional continuity construct that exists in stories published by comic book company DC Comics. The DC Multiverse consists of numerous versions of the DC Comics Earth, allowing writers the creative freedom to explore alternative versions of characters and their histories without contradicting the continuity of the main universe. The number of alternate universes used by the Multiverse construct has varied over the years due to DC Comics' policy of using or abandoning the concept at various points in its publishing history.
Originally, there was no consistency in spelling regarding "numbered" Earths — they would be either spelled out as words or use numbers even within the same story (such as was the case with Earth-3 in 1964) but a tradition of spelling them out developed. Because the current Multiverse (brought back via Infinite Crisis and 52) uses numbers some people mistakenly believe that this was a way DC separates the current multiverse from the "original" one.
Read more about Multiverse (DC Comics): List of Universes, Contact Between Universes, Other Versions, Other Media