(set theory) A function of "many-to-one" mapping relationship; more formally, f: X → Y is a surjection if and only if, for every y in the codomain Y, there is at least one x in the domain X with f(x) = y.
Examples
The function s is not a surjection because it never takes on an odd value.
Note in particular that a function is a bijection if and only if it's both an injection and a surjection.