counterfactual (adj.)

word-forming element used in English from c. 1300 and meaning "against, in opposition; in return; corresponding," from Anglo-French countre-, French contre-, from Latin contra "opposite, contrary to, against, in return," also used as a prefix (see contra (prep., adv.)). A doublet of contra-. In some cases it probably represents a purely English use of counter (adv.).

1834, formed from fact on model of actual. Related: Factually.