Define a function as a correspondence between two sets.
A function from a set X to a set Y is defined as a correspondence between sets X and Y such that for each element of X, there is only one corresponding element in Y.
The set X is called the domain of the function.
The set Y is called the range of the function.
For example, X = {a, b, c}, Y = {1, 2, 3, 4, 5} and f be a correspondence which assigns the position of a letter in the set of alphabets.
Therefore, f(a) = 1, f(b) = 2 and f(c) = 3.
As there is only one element of Y for each element of X, f is a function with domain X and range Y.