Parallel universe

A parallel universe (or an alternate universe) is a separate and self-contained universe existing as consequence of different choices and outcomes than those that have led to our own state of reality. An infinite number of parallel universes exist, one for every possible outcome of every possible event that occurs throughout time.

For example, there exists a parallel universe in which Jean-Luc Picard was lost to the Borg, and another where the Borg triumphed over the Federation. In another, Worf married Deanna Troi. (TNG: "Parallels")

The most frequent contacts between "our" universe and a parallel universe involved the so-called mirror universe, with the first reported crossover occurring in 2267. (TOS: "Mirror, Mirror")

Alternate timelines differ from parallel universes. An alternate timeline is created by time travel within the same universe, spawning a different set of historical events. Multiple timelines are a result of time travel, which can both alter and restore the timeline.

Many Star Trek fan-fiction venues are, by their nature, set in parallel/alternate universes. Following are some examples:
 * ARC undefined

Canon references

 * ENT:
 * "In a Mirror, Darkly"
 * "In a Mirror, Darkly, Part II"
 * TOS:
 * "Mirror, Mirror"
 * "The Tholian Web"
 * TNG:
 * "Parallels"
 * DS9:
 * "Crossover"
 * "Through the Looking Glass"
 * "Shattered Mirror"
 * "Resurrection"
 * "The Emperor's New Cloak"