Alternate history or alternative reality is a genre of fiction consisting of stories that are set in worlds in which one or more historical events unfolds differently than it did in the real world. It can be variously seen as a sub-genre of literary fiction, science fiction, and historical fiction; different alternate history works may use tropes from any or all of these genres. It is sometimes abbreviated AH. Another occasionally used term for the genre is "allohistory" (literally "other history"). See also Fictional universe.

Since the 1950s, this type of fiction has to a large extent merged with science fictional tropes involving cross-time travel between alternate histories or psychic awareness of the existence of "our" universe by the people in another; or ordinary voyaging uptime (into the past) or downtime (into the future) that results in history splitting into two or more time-lines. Cross-time, time-splitting, and alternate history themes have become so closely interwoven that it is impossible to discuss them fully apart from one another. "Alternate History" looks at "what if" scenarios from some of history's most pivotal turning points and presents a completely different version, sometimes based on science and fact, but often based on conjecture. The exploration of how the world would look today if various changes occurred and what these alternate worlds would be like forms the basis of this vast subject matter.

In French, Italian, Spanish, and German, the genre of alternate history is called uchronie / ucronía, which has given rise to the term Uchronia in English. This neologism is based on the prefix u- (as in the word Utopia, a place that does not exist) and the Greek for time, chronos. A uchronia, then, is defined as a time that does not exist, a "non-time".