A function cannot have multiple outputs (y-values) for any given input
(x-values). For example, y=2x+3 is a function because no mater what
number x represents, there is a single, unique y-value (When x=3 y=9 ).
In other words, you can never have two (or more) y-values that
correspond to a single x-value. Therefore, a vertical line, for example
x=2, cannot be a function because it has an infinite amount of y-values
for a single x-value. A horizontal line is a function
because there is only one y-value for any given x-value. For example,
y=3. No mater what x-value is used, there is only one y-value possible
(in this case, y will always equal 3). I am, of course,
referring to the cartesian coordinate system, also known as the
rectangular coordinate system
