"Earth 616" is the designation that was given during the Dave Thorpe/Alan Moore Captain Britain stories to the Earth where the "real" Marvel Universe resides. I believe Moore has alternately taken credit for the number and said that Thorpe coined it. To my recollection, I don't recall it appearing in a Thorpe story, but it's possible I'm wrong. At any rate, I really feel that if it weren't for its association, deserved or otherwise, with Alan Moore, "616" would not see nearly as much use as it does.
But the point is, the term bothers me because it just sounds silly -- I (figuratively) roll my eyes when I see or hear someone refer to the "main" Marvel Earth as "Earth 616," "616," or, stupidest of all to my ear, "the 616." Just call it the Marvel Universe! No one will stop and ask you if you mean Earth 12, Earth 457, or Earth 8532. I promise we all know exactly which Earth you're talking about. Trust me.
Even if fandom doesn't necessarily agree with my above sentiment, I'm gratified that Marvel does. I've said before that I have little use for modern Marvel, but one area where we see eye-to-eye is the stupidity of "616". Several of Marvel's editors, from Joe Quesada on down, have stated their distaste for the term, as well as their confusion as to why anyone would even want to use it.
So: sorry if I've offended anyone who uses the number. This is only my personal opinion. You will never see the term "616" used on this blog beyond this post. And yet somehow, even without that indicator, you will still know which universe I'm referring to at any given time.