The many alternate universes of Gravity Falls.
An Alternate Universe is a self-contained reality co-existing with one's own. A specific group of parallel universes are called a "multiverse," although this term can also be used to describe the possible parallel universes that constitute reality.
Description[]
While the terms "parallel universe" and "alternative reality" are generally synonymous 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. The term "parallel universe" is more general, without any connotations implying a relationship or lack of relationship, with our own universe.
Overview[]
According to W. D. Gaster and Rick Sanchez, alternate universes are more like "what if?" scenarios. Basically, a universe that is attributed to the main one but with different results should be the choices that the characters make become drastically different.
It can also be a universe where the roles of the characters are different as well. For example, there is a universe where Dipper and Mabel's personalities are switched, a universe where Sans and Papyrus become humans and the humans become monsters, a universe where Coraline becomes the Beldam, and even a universe where all the protagonists become antagonists and vice versa.
Alternate universes can also be somewhat related to time as Gaster states that every single choice or action they make in their main universe consequently creates an alternate reality where they did or did not make that specific action or choice. The only people who know most about these alternate realities are Gaster, Rick Sanchez, Sans, The Doctor, Elizabeth, and Stanford Pines; although only three have actually seen and visited these realities. Sans is aware of these alternate universes, though he chooses not to indulge himself in these sorts of things.