A **binary function** is a function where the domain is a subset of the Cartesian product of two sets. For example, the function *plus* : **N** × **N** `->` **N** that maps an ordered pair of natural numbers to their sum. The application of *plus* to an ordered pair (*x*, *y*), that is *plus*((*x*, *y*)) is usually simply written as *plus*(*x*, *y*).

See also binary operation.