Source : Free On-Line Dictionary of Computing

delta reduction
     
         In {lambda-calculus} extended with constants, delta
        reduction replaces a function applied to the required number
        of arguments (a {redex}) by a result.  E.g. plus 2 3 --> 5.
        In contrast with {beta reduction} (the only kind of reduction
        in the {pure lambda-calculus}) the result is not formed simply
        by textual substitution of arguments into the body of a
        function.  Instead, a delta redex is matched against the left
        hand side of all delta rules and is replaced by the right hand
        side of the (first) matching rule.  There is notionally one
        delta rule for each possible combination of function and
        arguments.  Where this implies an infinite number of rules,
        the result is usually defined by reference to some external
        system such as mathematical addition or the hardware
        operations of some computer.  For other types, all rules can
        be given explicitly, for example {Boolean} negation:
     
        	not True  = False
        	not False = True
     
        (1997-02-20)