Noun

1. A claim, hypothesis, or other belief that is contrary to the facts.
2. A conditional statement in which the conditional clause is false, as "If I had arrived on time . . .".
   • 1952, B. J. Diggs, "VI.—Counterfactual Conditionals," Mind, vol. 61, no. 244, p. 513:

     In recent years there has been increasing discussion of the "problem of counterfactuals". One way of formulating this problem is as follows: "What is meant when one asserts a conditional statement, the antecedent of which is contrary to fact?"