 In a Church-Rosser system, an object has "at most one" normal form; that is the normal form of an object is unique if it exists, but it may well not exist.In lambda calculus for instance, the expression (λx.xx)(λx.xx) does not have a normal form because there exists an infinite sequence of beta reductions (λx.xx)(λx.xx) → (λx.xx)(λx.xx) → ...Various properties, simpler than Church-Rosser, are equivalent to it.