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.
I'll take that bet.
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.
The concept may exist, but so does the concept of "air" - that doesn't mean it should get an entry. My point is that we shouldn't be defining Superman's 1938-1986 reality using terms that where created specifically for
Crisis - an event designed to destroy that reality. The reality should define itself, not be defined by it's destroyer. I agree with Telle, that we can get by with the occasional mention of a parallel universe or parallel world, and maybe Earth-1 and Earth-2.
We seem to just be going back and forth on this, and I think that if we can't reach common ground we should ask the Great Rao to step in.