In mathematics, an injective function is a function f that maps distinct elements of its domain to distinct elements; that is, x1 ≠ x2 implies f ≠ f . = f implies x1 = x2 in the equivalent contrapositive statement.) In other words, every element of the function's codomain is the image of at most one element of its domain. The term one-to-one function must not be confused with one-to-one correspondence that refers to bijective functions, which are functions such that each element in the codomain is an image of exactly one element in the domain.