A vertical line is a line that runs parallel to the $y\text{-}$axis. A vertical line is drawn in the coordinate plane below.

Notice that the line passes through all points where the $x\text{-}$coordinate is $3.$ This means that the same $x\text{-}$value corresponds to infinitely many $y\text{-}$values. Therefore, this line does not represent a function. In fact, no vertical line is a function. Similar to horizontal lines, vertical lines take the following form.

Here, $a$ is the $x\text{-}$intercept. The rule for the line above is $x=3.$