A multiverse is a set of a finite or infinite number of universes, frequently interpreted as a space containing distinct, typically non-interacting universal "bubbles". Occasionally, the term is used to define the hypothetical set of every single possible universe, including the universe in which we live.
A number of distinct multiverse hypotheses exist that can describe multiverses in various ways, but in essence the feature in common is that they involve multiple universes or realities distinct from each other and from the one that we, or at minimum a single observer, experiences.
The various universes within a multiverse are called "parallel universes,” "other universes,” or "alternate universes.” It is possible to classify a multiverse based on what types of universes it contains and how they relate to each other (see Altverse).
The dimensionality of a multiverse depends on the specific interpretation of the multiverse taken.
A simple conception of a multiverse is five dimensional, with several four dimensional universe-"lines" arranged in a stack in a five dimensional space.
One can also construct a multiverse with the same dimensionality as the universes it contains; if the universes are finitely large, they can simply sit inside a larger space like bubbles in a volume of water. Provided that the multiverse is negatively curved, one can even fit infinitely many infinitely large universes of the same dimensionality inside. In this model, each infinite universe is a horohyperball bounded by a horohypersphere. Using a Poincaré hyperball model to model the geometry of such a multiverse, every representation of an infinite universe would be a hyperball tangent to the outer boundary of the unit hyperball.
The most extreme conception of the multiverse has them be infinite dimensional, with the coordinates of a point in the multiverse representing the complete state of the universe located at that position (such a multiverse can also be called a phaseverse).
Multiverses as Generalization
A feature common among many cosmologies is to take a multiverse as a generalization or abstraction of the universes it contains: for example, whereas a single universe might have a single mathematical basis for its laws of physics, a fixed number of dimensions, a single set of fundamental constants, and an initial condition, a multiverse can be constructed in which every universe has three of these four held constant but the other is free to vary between universes. One operation by which this may be done is the power-set, which has the useful property in that the "size", in one sense, of a multiverse containing infinitely large universes is one beth number larger than that of those universes.
In this model, it then becomes feasible to introduce a megaverse as a structure that behaves like a "multiverse of multiverses", perhaps losing another feature in common among the universes it contains, and to continue ad infinitum in what is called an archverse chain.
While his writing is inconsistent with the description given above, it is nonetheless common for discussions of the multiverse to bring up the work of physicist Max Tegmark, who proposes that cosmology may be layered up to four levels deep, each one requiring fewer assumptions about physics than before. As such, these levels are outlined below for whatever reference purposes they may be needed for.
A Type I multiverse, the least like a multiverse as written above, is a natural consequence of an infinite, isotropic, homogenous universe: if the probability of a cosmological region - be it a cubic centimeter or a Hubble volume - having certain contents in a certain arrangement after the Big Bang is nonzero, then an infinite universe would contain infinitely many such regions. Popularly, this argument is taken to mean that an exact copy of our own observable universe exists somewhere else in the universe and that identical regions of this size are spaced apart by approximately meters.
A Type II multiverse would instead be a consequence of the chaotic inflationary theory of cosmology, which purports that quantum fluctuations shortly after the Big Bang created mutually inaccessible pockets of space that, having evolved differently from the unified high-energy physics before them, may have ended up with different physical constants or even geometries. In the nested interpretation, each pocket could itself be a Type I multiverse. Such an argument is sometimes invoked to explain apparent cosmological fine-tuning, in which an infinite number of different, perhaps inhospitable, universes exist alongside our own so that our physical constants thus come across as less extraordinary.
A Type III multiverse would be a consequence of the many-worlds interpretation of quantum mechanics, in which quantum events with probabilistic outcomes resolve in all ways at once, passing into a vast number of Everett branches each considered to be its own universe. Such a multiverse would function identically to a Type I, only in alternate quantum branches rather than alternate locations in 3-space; one could model a divergence as either a single branch that splits into two, or as a pair of Hubble volumes in the Type I multiverse which begin to differ only upon the resolution of one particular quantum function.
A Type IV multiverse abstracts even further from the above, positing that alternate spaces with different general principles of physics and mathematics exist independently of ours. Tegmark asks "why these equations and not others?" and resolves the question with the assumption that any other set of equations also exists as a distinct mathematical - and thus physical - structure. This may be analogous to a megaverse or gigaverse, in which one needs even fewer starting assumptions than a set of laws of physics.
Multiverse and Omniverse
It is common, especially in fiction, to see the terms "multiverse" and "omniverse" used interchangably to describe a set of universes, often specifically written as the set of all possible universes or all fictional universes. On this wiki, the omniverse is taken to be some number of levels above the multiverse, perhaps infinitely many, but neither one is seen as an insurpassable boundary. Because fiction can be written involving structures beyond a multiverse or omniverse, interpreting either as a container of all fiction, human thought, or otherwise is incorrect by this view.