A * mapping* *M: D &rarr R* specifies
a correspondence between elements of a * domain*
*D* and a * range* *R*.

If each element of *D* maps to a single element of *R*, the mapping
is * one-to-one* or * injective*
.

If every element of *R* is mapped to by some element of *D*, the
mapping is * onto* or * surjective* .

A mapping that is both one-to-one and onto is * bijective*.