Other names for this idea are counterfactualism, virtual history, and uchronia, though the most common term is alternative history (alternate history in the US).
It’s a “what if” approach, which works out what might have happened if some nodal event in history had not occurred or had turned out differently — what if Napoleon had won at Waterloo, for example (it was “a damn close-run thing”, you may recall). It has long been a staple plot type for science fiction, from Ward Moore’s Bring the Jubilee, through Philip K Dick’s The Man in the High Castle and Keith Robert’s Pavane, to Orson Scott Card’s Pastwatch and Kim Stanley Robinson’s recent The Years of Rice and Salt.
In scholarly historical circles, “what if” speculation has in the past been unpopular to the point of derision, though this is now changing, in particular in the field of military history. The British historian Niall Ferguson wrote a book in 1997 in which he defended allohistory — he argued that if the study of history is ever to be able to predict future events on the basis of past ones, it is important to analyze what might (or should) have happened, as well as what actually did.
The prefix allo- is from Greek allos, “different, other”, as in allegory and allergy.
In Virtual History, he debates those of his colleagues who dismiss allohistory as mere science fiction.
Sydney Morning Herald, March 2002