Bijection

A bijection is a function f : X -> Y which maps the domain X to the range Y

the function f is:

1. injective (or one-to-one), that is, for every y in Y there is at most one x in X such that f(x) = y, and
2. surjective (or onto), that is, for every y in Y there is at least one x in X such that f(x) = y.