call-by-need

    A reduction strategy which delays evaluation of
   function arguments until their values are needed.  A value
   is needed if it is an argument to a primitive function or it
   is the condition in a conditional.  Call-by-need is one
   aspect 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)