The Omniverse is the ensemble of all possible universes, with all possible laws of physics.
Hierarchy within the omniverse
Universe: The inside description of a context that is relative in size/structure (attributes/modes) to the known universe that we inhabit. A Universe, also known as a Cosmos, is a particular individual space-time organization with a specified number of dimensions of space and time and definite and specific laws of physics. Other Universes (other Cosmoses) may have different numbers of dimensions of space and time and different laws of physics than our own Universe (Cosmos).
Multiverse: The part of infinity that directly joins a given universe with all possible configurations of that universe.
Metaverse: In string theory, the part that is along with, after; over also denoting change in the multiverse that houses the branes or film that each universe is said to be attached to and hang like individual sheets in a hypermagnetic wave with rhythms of hypercosmic strings going up and down that has a third element causing up, down, backwards, forwards, motions inside the Xenoverse.
Xenoverse: the unknown alien elements that are beyond and part of the metaverse and multiverse structure. Compared to a patchwork quilt hanging on a line to dry in space that is multivariate inside the Omniverse. While Omniverse is said to be the outside ring of all that is known, the xenoverse is the inside the hypermacrocosm that is unknown beyond the metaverse—the unknown sets of laws that govern how branes behave to create metaverses, the laws of which govern the creation of multiverses.
Omniverse: All possible attributes and modes are in play, multiverses are categorized by the attributes/modes active in its child universes. Some or all possible modes of existence are actualized. If we take the point of origin as our being as a point in measurement, then we can generate the following hierarchy: 1. our location in space-time, 2. this universe (cosmos), 3. the multiverse, 4. the metaverse, 5. the xenoverse, 6. the omniverse.
Allegorical illustrations of the concept
One can think of the omniverse as a tree structure: the omniverse is the trunk, the metaverse is the laws that govern the formation of branches, each multiverse is a branch, and each universe (cosmos) is a leaf.
Alternatively, the omniverse can be illustrated as a forest, a metaverse is the laws that govern the cosmic ecology that determines the distribution of trees in the forest, a multiverse is a tree in the forest, a universe as a branch on that tree, and all further branches and leaves are further subset horizons within that universe.
Physicists such as Stephen Hawking and Roger Penrose have suggested that universes both fork and combine, which could be visualized as more of a system of roads and pathways.
In other words
Think of it like this: the omniverse is basically everything. Every universe, every dimension. Everything. The omniverse is made up of countless multiverses and each multiverse has its own set of rules. Different universe, different rules. Magic A from Universe A might work differently in Universe B et cetera. Magic is stronger in the DC Multiverse while weaker in the Marvel Multiverse. Each multiverse also has its own unique dimensions. The DC Multiverse has the Phantom Zone and the Speed Force while the Marvel Multiverse does not.
There are various powers and abilities that will outside of their home universe and those that do not.
Basically, be careful when traveling around the omniverse.
Real life history
The term "omniverse" is believed to have been created by Mark Gruenwald. It has been popularized by the composer-writer Sun Ra.