Originally posted by operator616
Marvel hasn't always used them interchangeably. Quasar, New exiles, and several handbooks have all mentioned that the omniverse encompasses the multiverse. That and the fact marvel has referenced (and even depicted) several times several multiverses (FF annual, Marvel Knights 4, Quasar, hell even Cable's title mentioned it and i recall also a marvel UK title mentioning it as well.)The term actually originated before Captain Britain comics from what i recall. I remember a dr strange comic in the late 70s mentioning the term omniverse and also a thor letter's page in that time era. But back then it just wasn't properly defined yet.
Anyway, it's actually a very well established fact that Marvel does have multiple multiverses, so differentiating between the terms seems only logical, wouldn't you say?
I think differentiating would be logical if there was internal consistency, but Marvel have always been happy for both multiverse and omniverse to mean the entirety of existence. Especially as even at the time of the stories that specifically mention omniverse there are others that use multiverse in the same way.
You can probably make cases out of specific examples and character statements, but they basically mean the same thing at this point. If it needs an explanation then I see it that if multiverses within the context of the omniverse are mentioned, you could sub that in with specific collections of universes within the multiverse, or vice versa.