Satisfying properties (1) and (2) means that a pairing is a function with domain "X". It is more common to see properties (1) and (2) written as a single statement: Every element of "X" is paired with exactly one element of "Y". Functions which satisfy property (3) are said to be "onto "Y" " and are called surjections (or "surjective functions"). Functions which satisfy property (4) are said to be "one-to-one functions" and are called injections (or "injective functions"). With this terminology, a bijection is a function which is both a surjection and an injection, or using other words, a bijection is a function which is both "one-to-one" and "onto".