Alternate history is a type of science fiction in which the basic premise is that some specific historical event never happened, or happened differently. Currently the most prolific practitioner of this type of fiction is Harry Turtledove, who has written a series in which the South did not lose the American Civil War. Other stories by this author include the premise that America had not been colonised from Asia during the last Ice Age; as a result, the continent still has living mammoths and prehuman species. See also steampunk.
The earliest example of alternate history appears to be Book IX, sections 17-19, of the Livy's History of Rome from Its Foundation. He contemplates the possibility of Alexander the Great expanding his father's empire westward instead of east, and attacking Rome in the 4th century BC. (Wikipedia contains spoilers: Livy was a patriotic Roman -- Alexander loses.)
Many people one would not consider sci-fi authors have written alternate history. In "The Forfeited Birthright of the Abortive Far Western Christian Civilization," Arnold Toynbee describes a world in which the Franks lost to the Muslims at the Battle of Tours in 732. Winston Churchill wrote an essay entitled "If Lee Had Not Won the Battle of Gettysburg" that considers what sort of world would have resulted if the North had won the American Civil War -- from the point of view of a historian in a world where the Confederacy had won.
Historians also speculate in this manner; this type of speculation is known commonly as counterfactuality. There is considerable debate within the community of historians about the validity and purpose of this type of speculation.
- Pavane, by Keith Roberts, assumes that Queen Elizabeth I was assassinated, and in the ensuing disorder, the Spanish Armada was successful in suppressing Protestantism; the novel (actually a series of shorter pieces) is set in a 20th century where technology has advanced less than in our world, and where the Inquisition still has power.
- The Alteration by Kingsley Amis is set in a world very similar to that of Pavane; the novel concerns the attempt to prevent a young boy with a perfect singing voice from being recruited to the Vatican's eunuch choir. There are a number of in-jokes, where famous works of fantasy and science fiction appear, under slightly different titles: `The Wind in the Cloisters' and `The Lord of the Chalices' for example.
- The `Lord Darcy' fantasy series by Randall Garrett; a number of short stories and one novel (Too Many Magicians) based on the premise that King Richard I of England returned safely from France and that Roger Bacon had systematised the laws of magic. The stories are a series of traditional detective fiction-style murder mysteries with forensic magic being used in the investigation.
- See Alternate Earths (ISBN 1-55634-318-3) and Alternate Earths II (ISBN 1-55634-399-X) for several alternate history scenarios each.
- The Difference Engine by William Gibson and Bruce Sterling is a steampunk novel which deals with a Victorian society in which Charles Babbage's Analytical Engine becomes available a century early.