One-to-One
A function is one-to-one if different inputs yield different outputs. More formally, a function f is one-to-one if for each pair of different inputs a and b in the domain of f, the output values f(a) and f(b) are different.
A function is one-to-one if and only if its graph passes the horizontal line test.
A function is one-to-one if and only if it has an inverse.