The vertical line test is used to determine if a graph of a relationship is a function or not. if you can draw any vertical line that intersects more than one point on the relationship, then it is not a function. The vertical line test can be used to determine whether a graph represents a function. If we can draw any vertical line that intersects a graph more than once, then the graph does not define a function because a function has only one output value for each input value. Cutting or Hitting the Graph at Exactly One Point
If a vertical line intersects the graph in some places at more than one point, then the relation is NOT a function. Here are some examples of relations that are NOT functions because they fail the vertical line test.