Injective function
An injection (or one-to-one function) is a function f: X -> Y which maps the domain X to the range Y such that, for every y in Y there is at most one x in X such that f(x) = y. Put another way, given x and x' in X, if f(x) = f(x'), then it could only be because x = x'.
see also: Surjection and Bijection