- Parallel universes in fiction (Wikipedia)
A parallel universe, also known as an alternate universe, parallel world, parallel dimension, alternate reality, or alternative dimension, is a hypothetical self-contained layer or plane of existence, co-existing with one’s own. While the six 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.