function

Let A and B be two non-empty sets. Then a function "f" from set A to set B is a rule or method or correspondence which associates elements of set A to elements of set B such that

(1) all elements of set A are associated to elements in set B.
(2) an element of set A is associated to a unique element in set B.

In other words, a function "f" from set A to set B associates each element of set A to a unique element of set B.