A multiverse is a set of a finite or possible 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).
Dimensionality
The dimensionality of a multiverse depends on the specific interpretation of the multiverse taken.
A simple conception of a multiverse is 5-Dimensional space, with several four dimensional universe-"lines" arranged in a stack in a 5 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.