Answer:
a function if no vertical line has more than one point of intersection
Explanation:
Examine the graph to see if any vertical line will intersect it at more than one point. This will be the case if individual points are plotted one above the other, or if there is a curve that folds on itself horizontally (as a parabola that opens sideways, for example). Of course, closed curves such as a circle or ellipse will also have multiple intersection points with a vertical line.
If a vertical line intersects the graph in more than one place, the "vertical line test" fails. If the test fails, the relation is not a function. If the test passes (there are one or fewer points of intersection with any vertical line), then the relation is a function.