In Metaversalism's usage of the word, the Metaverse (which we also sometimes call the Omniverse, so as not to confuse it with pre-existing senses of the word "Metaverse") is a Platonic, metaphysical entity comprising the pure, abstract structure of all mathematically-definable entities, including all possible universes, conscious souls, and gods that could be (at least in principle) identified by a sufficiently complex and detailed mathematical description.  (Even infinite descriptions might be allowed, although the Church of Metaversalism has not yet taken a definitive position on this question.)

The most fundamental premise of Metaversalism is its position that the Metaverse exists (in a mathematical sense) because it must exist, by pure logical necessity; that the Metaverse contains everything that could exist; and that no more explanation than this is required to explain why our universe exists, or to answer the question, "Why does anything exist at all?"

To see why the Metaverse must exist, suppose nothing existed at all.  But even then, it would still be true that if any hypothetical sentient were to postulate Peano's axioms of arithmetic, then all the properties of the natural numbers 1, 2, 3... would follow.  Thus, the entire structure of the natural numbers already exists in the logical implications of this conditional - and this is true even if the "if" part of the conditional is not satisfied, because there is nobody there to postulate the axioms.  The structure of the conclusions that would follow is still there; it exists independently of anything.  The same goes for all other mathematically definable objects and structures.  The concept of everything arising from nothingness in this way is evoked by our Latin motto "Ex nihil panton" (or, new version, "Ex nihilo omnia fiunt") in Metaversalism's logo.

To see why mathematical existence ought to be enough for everyone...  According to the philosophy embodied by modern physics, the detailed structure of our universe is identical to that of some complex mathematical structure (whose underlying definition is not fully known yet).  This structure contains all the rich details of how galaxies, stars, and planets form, how life arises and intelligence evolves, how the mind functions.  Its rich tapestry includes all of the thoughts, feelings, and emotions that we experience.  Even if mathematical existence is the only kind of existence there is (Metaversalism's position), the rich, evolving structure that as a human consciousness still cannot not tell the difference between this and any other kind of existence - because all the details of that mind's activity and experiences are already contained there in the mathematical structure.  Having another kind of existence would not change any of the details of that conscious structure, so would not be perceptible.

Note: The concept of the Metaverse described above is distinct from the "multiverse" concept that is encountered in discussions of the "many-worlds" or "decoherent histories" interpretations of quantum theory.  In the traditional quantum multiverse, all of the universes/histories still obey quantum theory internally.  But our Metaverse also includes universes that are not based on quantum mechanics at all, as well as mathematical structures that would not usually even be considered to represent universes.  Our use of the term "Metaverse" is thus also entirely distinct from its use in science fiction and online to denote certain 3-dimensional virtual reality systems (such as that of Second Life).  Our Metaverse is a much broader concept, and one that, we believe, is more deserving of the prefix "meta".