Beta normal form

In the lambda calculus, a term is in beta normal form if no beta reduction is possible. A term is in beta-eta normal form if neither a beta reduction nor an eta reduction is possible. A term is in head normal form if there is no beta-redex in head position. The normal form of a term, if one exists, is unique (as a corollary of the Church–Rosser theorem). However, a term may have more than one head normal form.