One-one
One-one
A relation R is one-many if for every y in the converse domain there is a unique x such that xRy. A relation R is many-one if for every x in the domain there is a unique y such that xRy. (See the article relation.) A relation is one-one, or one-to-one, if it is at the same time one-many and many-one. A one-one relation is said to be, or to determine, a one-to-one correspondence between its domain and its converse domain. — A.C.