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).