A surjection, or surjective function, is a type of function in mathematics where every element in the target set is mapped to by at least one element from the domain. This means that for every output value, there is at least one input value that produces it. Surjective functions ensure that no element in the target set is left out. In simpler terms, if you think of a surjective function as a way to connect two groups, every member of the second group must have a connection to at least one member of the first group. This concept is important in various areas of mathematics, including set theory and function analysis.