A conditional statement whose antecedent [see below] is known (or, at least, believed) to be contrary to fact. Thus, for example, “If George W. Bush had been born in Idaho, then he would never have become President.”
Unlike material implications [see below], counterfactuals are not made true by the falsity of their antecedents. Although they are not truth-functional [see below] statements, counterfactuals may be significant for the analysis of scientific hypotheses.
[antecedent – The element that states the prior condition in any conditional statement. For example, “It doesn’t rain” is the antecedent in both
“If it doesn’t rain, then we’ll have a picnic.” and
“It will reach ninety degrees today if it doesn’t rain.”
(Via Philosophical Dictionary)