Bijection

A mapping that is both one-to-one (an injection) and onto (a surjection), i.e. a function which relates each member of a set S (the domain) to a separate and distinct member of another set T (the range), where each member in T also has a corresponding member in S.

i.e.,

A bijective function, f: X → Y, where set X is {1, 2, 3, 4} and set Y is {A, B, C, D}.

For example,

f(1) = A

f(2) = C

f(3) = B

f(4) = D