The Omniverse holds all of reality. Containing everything imaginable (or at least connected to).
An alternate definition of the Omniverse is that it contains an infinite number of these "layers"; universes combine to make multiverses, multiverses combine to make megaverses... and the stack, which can be called an infinite fractal array of archverses, keeps going forever. In this way the Omniverse could be a direction one approaches rather than a physical set containing some infinite number of -verses.
The Omniverse contains many different types of -verses, including these somewhat lesser-known ones:
The only trait every single thing within the Omniverse share is that it exists. How it exists varies in countless ways.
Uses in fiction
Despite the term 'Multiverse' being used far more often, the term and concept of the Omniverse can still be found in popular fiction:
Beyond the Omniverse
The Omniverse exists in the surrounding emptiness known as The Outside, a gigantic void of virtual nothingness. Whatever may lie outside of these concepts is simply referred to as Beyond. All of this however is of course still within The Box.