For questions in which reality is different than it is in real life. This may range from small, historical changes to absolute revisions of the laws of physics.
An alternate reality is a reality - the state of all things that exist, or have once existing - but altered for the purpose of fiction. This tag may refer to the reality being built if it is different from our own, or to a reality that coexists with the main one in the narrative.
Not to be confused with alternate-worlds, although that tag may also be relevant.