Counterfactual (adjective) : expressing what has not happened but could, would, or might under differing conditions – British Dictionary
Counterfactual (noun) : a conditional statement in which the first clause is a past tense subjunctive statement expressing something contrary to fact, as in: if she had hurried she would have caught the bus. – British Dictionary
In physics the word counterfactual does not mean “something contrary to fact.” Instead, it means that some physical property could have been measured but, for one reason or another, were not.
Counterfactual definiteness: an expression used by the supporters of determinism. Mark Alford explains it this way 
“In a deterministic theory, even for a measurement that was not actually performed there is a fact of the matter about what result it would have yielded (“counterfactual deﬁniteness”).”
Wikipedia article on counterfactual definiteness summarizes it as follows:
“In quantum mechanics, counterfactual definiteness (CFD) is the ability to speak meaningfully of the definiteness of the results of measurements that have not been performed (i.e. the ability to assume the existence of objects, and properties of objects, even when they have not been measured). The term “Counterfactual definiteness” is used in discussions of physics calculations, especially those related to the phenomenon called quantum entanglement and those related to the Bell inequalities.”