Counterfactual definiteness or CFD is a property of some interpretations of quantum mechanics but not others.
It refers to the ability to speak meaningfully about the results of measurements that were not performed.
For example, by the Heisenberg uncertainty principle, one cannot simultaneously know the position and momentum of a particle. Suppose one measures the position, this destroys any information about the momentum. The question then becomes, is it possible to talk about the measurement one would have received if one did measure the momentum instead of the position. In other words, if one conducted a different experiment, is there a single alternate time line that would have resulted from it?
CFD is a property of the Copenhagen interpretation of quantum mechanics as well as the hidden variables interpretation. It is not a property of the many worlds interpretation, and this allows many worlds to resolve Bell's inequalities without violating the locality principle.
See also