In mathematics, a function f is said to be surjective if its values span its whole codomain; that is, for every y in the codomain, there is at least one x in the domain such that f(x) = y .
Said another way, a function f: X → Y is surjective if and only if its range f(X) is equal to its codomain Y. A surjective function is called a surjection, and also said to be onto.
Reference:
http://en.wikipedia.org/wiki/Surjective_function
