Source : Free On-Line Dictionary of Computing

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)