The Free On-line Dictionary of Computing (30 December 2018):

negation by failure

   An extralogical feature of Prolog and other logic
   programming languages in which failure of unification is
   treated as establishing the negation of a relation.  For
   example, if Ronald Reagan is not in our database and we
   asked if he was an American, Prolog would answer "no".

   (1994-11-29)