Source : Free On-Line Dictionary of Computing

call-by-need
     
         A {reduction strategy} which delays evaluation of
        function arguments until they are needed because they are
        arguments to a {primitive} function or a conditional.
        Call-by-need is one part of {lazy evaluation}.
     
        The term first appears in Chris Wadsworth's thesis "Semantics
        and Pragmatics of the Lambda calculus" (Oxford, 1971, p. 183).
        It was used later, by J. Vuillemin in his thesis (Stanford,
        1973).
     
        (1995-05-27)