This page is about the closest I could find:
http://superman.nu/tales2/wife/?page=2There's a mention of parallel worlds in 'Superboy and the Five Legion Traitors' and a mention of alternate timelines in the issue where Superman's costume gets stolen. Alternate Earths were also mentioned in that story about Laney(The daughter of Lois Lane and Kal-El) when she accidently exposed herself to a box of Red K.
'Multiverse' seemed limited to Justice Leauge of America, Green Lantern, Flash. With the 'Wonder Woman: Judegment Day' miniseries being a recent addition at the time.
Earth X was mentioned in the pages of DC Comics Presents.
In Superman #276, Willy Fawcett mentions "fighting the Monster League of Evil across 1953 dimensions of time-and-space," and at the end of the story Superman refers to the "myriad earths that stretch across creation."
So the concept exist. I'll bet my right eye the term multiverse is used in the sources at least once, but who knows if we'll ever find it.
In any event the concept exists in the sources. Perhaps there's a more canonical name for it, but "myriad earths" doesn't have the same ring to it.