counterfactual
Concise Oxford English Dictionary © 2008 Oxford University Press:
counterfactual /kaʊntəˈfaktʃʊəl/ Philosophy
▶adjective relating to or expressing what has not happened or is not the case. ▶noun a counterfactual conditional statement (e.g. If kangaroos had no tails, they would topple over).
'counterfactual' also found in these Oxford entries:
