Alternate history or alternative history,〔''Brave New Words: The Oxford Dictionary of Science Fiction'' (Oxford University Press, 2007) notes the preferred usage of "Alternate History" as well as its primacy in coinage, "Alternate History" was coined in 1954 and "Alternative History" was first used in 1977, pp.4–5.〕 sometimes abbreviated as AH, is a genre of fiction consisting of stories that are set in worlds in which one or more historical events unfold differently from how they did in reality. It can be variously seen as a subgenre of literary fiction, science fiction, or 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 fiction 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 / ucronia / ucronía / 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''.
In writing an alternate history, the author makes the conscious choice to change something in our past. According to Steven H Silver, alternate history requires three things: 1) the story must have a point of divergence from the history of our world prior to the time at which the author is writing, 2) a change that would alter history as it is known, and 3) an examination of the ramifications of that change.
Several genres of fiction have been confused as alternate histories. Science fiction set in what was the future but is now the past, like Arthur C. Clarke's ''2001: A Space Odyssey'' or ''Nineteen Eighty-Four'', are not alternate history because the author has not made the conscious choice to change the past.〔 Secret history, works that document things that are not known to have happened historically but would not have changed history had they happened, is also not to be confused with alternate history.〔
Alternate history is related to but distinct from counterfactual history—the term used by some professional historians when using thoroughly researched and carefully reasoned speculations on "what might have happened if..." as a tool of academic historical research.
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