The more precise definition of the Vertical Line Test states:

If a graph, G, in the xy-plane has the property that there is at least one vertical line that contains more than one point on the graph, then there is no function y = f(x) whose graph is the graph G.

This precision is only needed when one looks at functions where x and y are the only only independent and dependent variables.  Other situations will be encountered in a Pre-Calculus or College Algebra course.

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