Alternate history or alternative history, sometimes abbreviated AH, is a genre of fiction consisting of stories that are set in worlds in which one or more historical events unfolds differently from how it did in reality. It can be variously seen as a subgenre of literary fiction, science fiction, and historical fiction; different alternate history works may use tropes from any or all of these genres. 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 into the past or into the future that results in history splitting into two or more timelines. 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, Catalan and German, the genre of alternate history is called uchronie / ucronía / ucronia / Uchronie, which has given rise to the term Uchronia in English. This neologism is based on the prefix ου- (which in ancient Greek means "not/not any/no") and the ancient Greek χρόνος (chronos), meaning "time". A uchronia means literally "(in) no time". This term apparently also inspired the name of the alternate history book list, uchronia.net.
All sources from Wikipedia.