# Surjective

A function $f$ from set $A$ to set $B$ is called a surjection and $f$ is said to be surjective if every element in $B$ is mapped onto by some element in $A$.

In symbols, $f$ is surjective if for every $y\in B$, there is some $x\in A$ with $f(x)=y$. (There may be more than one $x\in A$ with $f(x)=y$.)

A surjective function is also known as an onto function.