SVM Maximizing Margin
First of all, the formula given for the distance between a point and a line is not a definition, it is a calculation. The distance between a point $p$ and a line $\ell$ is the minimum distance between $p$ and a point on $\ell$.

Similarly, if $\ell,\ell'$ are two lines then the distance between them is the minimum distance between a point on $\ell$ to a point on $\ell'$. This is zero unless the lines are parallel, and if they are, say given by $x \bullet w + b = 0$ and $x \bullet w' + b' = 0$, then the distance is (I believe) $|b/\|w\| - b'/\|w'\||$. Again, this is not a formula but rather a calculation.