counterfactual

counterfactual \koun-ter-FAK-choo-uhl\, noun:

a conditional statement the first clause of which expresses something contrary to fact, as "If I had known."

The ruse is so obvious, a counterfactual posing as a home truth.
-- Matt Feeney, "Michael Chabon's Oakland," The New Yorker, September 26, 2012

Nevertheless, a counterfactual conditional differs from a piece of fiction only insofar as in the first case the addressee is requested to cooperate more actively in the realization of the text he receives...
-- Umberto Eco, The Role of the Reader

This word was born in the late 1940s from a portmanteau of two complete words. Counterfactual imagines a reality that is counter to the factual, or lived, experience.