A counterfactual, or a counterfactual conditional, is a conditional statement that possesses an if clause which is directly contrary to fact. A counterfactual also contains a then clause. Where the counterfactual differs from other conditionals is that it's then clause will only be certainly true if the counterfactual circumstance's if clause is true. The biggest and most notable examples of counterfactual thinking can be associated with statements such as "What if?" and "If only" and other statements that deal with a past that did not actually occur.
“What if Roosevelt had been assassinated in 1933? What if Spain had defeated the British navy in 1588? What if Napoleon had won the Battle of Waterloo? Such counterfactual premises are the basis for works of literature, often called ‘alternate histories’, that have gained in both popularity and scholarly attention in the past few decades.” - Kathleen Singles