a theoretical construct that could describe the alternative realities that may exit simultaneous with our own
This term is used to refer to Multi-Universes.
The totality of all the worlds and universes in the Multiverse Network.
A group of universes which are in some way related.
A multiverse (or meta-universe) is the hypothetical set of multiple possible universes (including our universe) that together comprise all of physical reality. The different universes within a multiverse are called parallel universes. The structure of the multiverse, the nature of each universe within it and the relationship between the various constituent universes, depend on the specific multiverse hypothesis considered.
In DC Comics, the Multiverse is a continuity construct in which multiple fictional versions of the universe exist in the same space, separated from each other by their vibrational resonances. Each universe in this multiverse varies from the others, in either subtle or profound ways. The universes are identified by referring to them as parallel Earths, such as "Earth-One", "Earth-Two", "Earth-Three", and "Earth-X".
The Multiverse is a series of parallel universes in many of the science fiction and fantasy novels and short stories written by Michael Moorcock. (Many other fictional settings also have the concept of a Multiverse.) Central to these works is the concept of an Eternal Champion who has potentially multiple identities across multiple dimensions.
Within Marvel Comics, most tales take place within the fictional Marvel Universe, this in turn is part of a larger multiverse. Starting with issues of Captain Britain, the main continuity in which most Marvel storylines take place was designated Earth-616, and the multiverse was established as being protected by Merlyn. Each universe has a Captain Britain designated to protect its version of the British Isles.
A few religious cosmologies involve ideas such as cyclic universes which can be interpreted as multiverses. A few of them even say that our universe is inside another universe.