Merriam-Webster defines "omniverse" as:
om·ni·verse \ ˈämnə̇ˌvərs, -və̄s, -vəis \ [plural -s] : a universe that is spatiotemporally four-dimensional...
To me, "The Omniverse" is the collection of every single universe, multiverse, megaverse, dimension (alternate or pocket) and realm. This includes not only the comic book universes of
Marvel Comics, but also
DC Comics (...with its "DC Rebirth Multiversity)",
Dark Horse Comics,
Boom Studios Comics,
Graphic India Comics,
Darby Pop Comics,
Valiant Comics, and every universe ever mentioned or seen (and an infinite amount never mentioned or seen) including our own world.
Everything is in the Omniverse, and there is only one Omniverse. According to the Official Handbook of the Marvel Universe: Alternate Universes. But it also includes every single literary work (From 11/22/63 by Stephen King to Zorba the Greek by Nikos Kazantzakis), television show (From 10 Things I Hate About You to Zoey 101), movie (From A (1998 Kannada Film to Zzyzx (2006)), urban legend (From 9/11 conspiracy theories to Yamashita's gold), universe, realm, etc. EVER.
It includes everyone from Popeye to Rocky Balboa to Ronald Reagan to Luke Skywalker to Snoopy to Jay and Silent Bob, etc. This includes universes outside of American and European western comics, such as Dragon Ball Z, Naruto, Bleach, One Piece, Ghost in the Shell and etc. eastern comic book media.
The Omniverse is EVERY reality, including those published by all other companies. Even fan-fictions, cancelled works, mere fantasies, wishes of thoughts created by people, future comic book publishing companies and fictional universes yet to be published are considered part of the Omniverse; simply put, The Omniverse is EVERY version of reality and existence imaginable. - Some Original material borrowed from Marvel Wiki and expanded.