Real Dictionary

counterfactual in American English
a counterfactual idea, assumption, or argument
contrary to the facts of an event, situation, etc.
noun: a conditional statement the first clause of which expresses something contrary to fact, as “If I had known”

counterfactual in British English
adjective: expressing what has not happened but could, would, or might under differing conditions
noun: a conditional statement in which the first clause is a past tense subjunctive statement expressing something contrary to fact, as in