Definition of "sf"

But some discussion of the complex relationship between “allohistory” and sf is appropriate here, as the genres overlap in certain ways. Classical allohistory— such as Trevelyan's "What if Napoleon had won the Battle of Waterloo?" and Churchill's "If Lee had not won the Battle of Gettysburg" —is a rigorously consistent thought-experiment in historical causality.

2011 February 1, Istvan Csicsery-Ronay, Jr., The Seven Beauties of Science Fiction‎, Wesleyan University Press, pages 102–103