Parallel universe (fiction)
A parallel universe or alternative reality is a hypothetical self-contained separate reality
In philosophy, reality is the state of things as they actually exist, rather than as they may appear or might be imagined. In a wider definition, reality includes everything that is and has been, whether or not it is observable or comprehensible...

 coexisting with one's own. A specific group of parallel universes is called a "multiverse
The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...

", although this term can also be used to describe the possible parallel universes that constitute reality. While the terms "parallel universe" and "alternative reality" are generally synonym
Synonyms are different words with almost identical or similar meanings. Words that are synonyms are said to be synonymous, and the state of being a synonym is called synonymy. The word comes from Ancient Greek syn and onoma . The words car and automobile are synonyms...

ous and can be used interchangeably in most cases, there is sometimes an additional connotation implied with the term "alternative reality" that implies that the reality is a variant of our own.