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Think of parallel universes as alternate histories or possibility. For example, in this universe, you have pancakes for breakfast, even though you could have something much healthier and won't lead you to diabetes. Parallel universes are basically those "what-if", those posibilities that you cast out because of your unhealthy diet, Because of this, parallel universes are nearly infinite in number, and all can sum up in a multiverse.
Matt Wright

A Parallel Universe, also known as Parallel Dimension, Alternate Universe, or Alternate Reality, is a hypothetical self-contained plane of existence, co-existing with one's own. There are an infinite universes that are identical to one another.

The sum of all potential parallel universes that constitute reality is often called a "multiverse". While the three terms are generally synonymous and can be used interchangeably in most cases, there is sometimes an additional connotation implied with the term "alternate universe/reality" that implies that the reality is a variant of our own, with some overlap with the similarly-named alternate history.

The term "parallel universe" is more general, without implying a relationship, or lack of relationship, with our own universe. A universe where the very laws of nature are different – for example, one in which there are no Laws of Motion – would in general count as a parallel universe but not an alternative reality and a concept between both fantasy world and earth.

Fiction has long borrowed an idea of "another world" from myth, legend and religion. Heaven, Hell, Olympus, and Valhalla are all "alternate realms" different from the familiar material realm. Plato reflected deeply on the parallel realities, resulting in Platonism, in which the upper reality is perfect while the lower earthly reality is an imperfect shadow of the heavenly. The lower reality is similar but with flaws.

Perhaps the most common use of the concept of a parallel universe is the concept of Hyperspace, the concept of "hyperspace" often refers to a parallel universe that can be used as a faster-than-light shortcut for interstellar travel.

Almost all beings have a Parallel counterpart except Celestial and Eldritch beings such as the Fae, Deities, Demons, Angels, Arcuthas, Outer Gods, Old Ones, and Primordials, as these beings exist on a higher plane of existence.