High School Mathematics - 2 4.1 Test of Functions: Vertical Line Test
The vertical line test
One way to tell if a graph is a function is the vertical line test, which states that if it is possible for a vertical line to meet a graph more than once, the graph is not a function.
Method to test a function:
Draw a line parallel to the y-axis. If this line cuts the graph at one distinct point, then it is a function otherwise it is not a function.