What is the X in X-mas?
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).