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.