A hypothetical, non-canonical situation in a fictional universe, such as exploring the implications of a deceased character still being alive, a faction winning a conflict they otherwise lost, or a radical shift in setting like fantasy to contemporary.
Some copyrights such as Undertale and OMORI are notable for having a large amount of fanmade alternate universes (AUs).
This tag should not be applied to copyrights such as Metal Gear and Fallout where an alternate timeline is part of the series' setting, unless it's intended to deviate from the established canon.
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