Vertical Line Test

In mathematics, the vertical line test is a test to determine if a curve is a relation or graph of a function when the function's domain and codomain correspond to the x and y axes of the Cartesian coordinate system. As a relation or graph of a function can only have one output for each unique input, such a Cartesian representation of the function can have at most a single y value for each x value. Thus, a vertical line drawn at any x position on the graph of a function will intersect the graph at most once.

INFO FOUND ON http://en.wikipedia.org/wiki/Vertical_line_test