Since the 1950s, this type of fiction has, to a large extent, merged with science fiction tropes involving time travel between alternate histories, psychic awareness of the existence of one universe by the people in another, or time travel 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.
In Spanish, French, German, Portuguese, Italian, Catalan and Galician, the genre of alternate history is sometimes 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 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.
The Collins English Dictionary defines alternative history as "a genre of fiction in which the author speculates on how the course of history might have been altered if a particular historical event had had a different outcome." According to Steven H Silver, an American science fiction editor, alternate history requires three things: a point of divergence from the history of our world prior to the time at which the author is writing, a change that would alter history as it is known, and an examination of the ramifications of that change.
Several genres of fiction have been misidentified as alternate history. Science fiction set in what was the future but is now the past, like Arthur C. Clarke's 2001: A Space Odyssey or George Orwell's Nineteen Eighty-Four, is not alternate history because the author did not make the choice to change the past at the time of writing. Secret history, which can take the form of fiction or nonfiction, documents events that may or may not have happened historically but did not have an effect on the overall outcome of history, and so is not to be confused with alternate history.
Alternate history is related to, but distinct from, counterfactual history. This term is used by some professional historians to describe the practice of using thoroughly researched and carefully reasoned speculations on "what might have happened if..." as a tool of academic historical research, as opposed to a literary device.
The earliest example of alternate (or counterfactual) history is found in Livy's Ab Urbe Condita Libri (book IX, sections 17–19). Livy contemplated an alternative 4th century BC in which Alexander the Great had expanded his empire westward instead of eastward; he asked, "What would have been the results for Rome if she had been engaged in a war with Alexander?" Livy concluded that the Romans would likely have defeated Alexander.
Another example of counterfactual was posited by cardinal and Doctor of the Church Peter Damian in the 11th century. In his famous work De Divina Omnipotentia, a long letter in which he discusses God's omnipotence, he treats questions related to the limits of divine power, including the question of whether God can change the past, for example, bringing about that Rome was never founded: