Originally posted by Omega Vision
I've always considered the term "Megaverse" superfluous. The way I see it there are Universes which make up Multiverses, Multiverses being the sum of all possible variations of a Universe and then there is the Omniverse which is the sum of all Multiverses.
I think megaverse was used in marvel to mean a cluster of universes or multiverses, whatever, with the same fundamental laws, such as the existence of time and space and things like that.
Multiverse is more often used for the alternate versions of a universe yes, and omniverse would be all the mega/multiverses.
But they're all pretty redundant, poorly defined and interchangeable with no clear cannon on their use. They all express the same basic idea of a population of universes.