A function is essentially a map between #x# values and #y# values. That is, for any given #x# value there must be at most one possible #y# value. The set of #x# values for which there is a corresponding #y# value is called the domain of the function.
In our example, the only #x# value for which there are any #y# values is #x=15#, and then the #y# value is totally unrestricted. For example, both #(15, 0)# and #(15, 1)# belong to the set of #(x, y)# values satisfying the equation.
So #x=15# does not define a function.
Another way of describing the condition above is that #x=15# fails the vertical line test - the requirement that any vertical line intersects the graph in at most one point. This fails when #x=15# because a vertical line at that point intersects the graph at infinitely many points.
