Vertical Line Test
We use the vertical line test to determine if a relation that is graphed on a coordinate plane is a function. If there are no vertical lines intersecting the graph of the relation at more than one point, then the relation is a function.
Another way to say this is if all possible vertical lines intersect the graph at exactly one point, then the graph of the relation is a function.
Below is an example of a graph of a relation that is also a function because it passes the vertical line test.