n (set theory) A function that maps distinct x in the domain to distinct y in the codomain; formally, a f: X → Y such that f(a) = f(b) implies a = b for any a, b in the domain.
n (mathematics) A relation on sets (X,Y) that associates each element of Y with at most one element of X.
n (figuratively) The addition of money to someone, or to a business.