On Wednesday last week, her column concerned "alternative history" or what is sometimes called counter-factual history. It's true that a few rather far-fetched counter-factual histories gave the genre a bad name, but in one sense counter-factual history is like comparative history: we (historians and everyone else) engage in them all the time on a casual basis, almost as a reflex. It's hard to imagine arriving at any new insight into past events without either measuring those events against other events we know about or speculating about what might have happened instead, had certain (apparently causal) events taken a different course.
Cynthia Crossen, "Déjà Vu: 'What Ifs' Don't Thrill Historians, but They Raise Intriguing Issues," Wall Street Journal, natl. ed., 2/2/05, B1.