What is the focal width of a parabola?

This is the length of the focal chord (the "width" of a parabola at focal level).

Let $x^2=4py$ be a parabola. Then $F(0,p)$ is the focus. Consider the line that passes through the focus and parallel to the directrix. Let $A$ and $A'$ be the intersections of the line and the parabola. Then $A(-2p,p)$, $A'(2p,p)$, and $AA'=4p$.

(In plainer English) Imagine a regular $x^2$ parabola. It is facing up, and the vertex is at $(0,0)$. Now, imagine a line parallel to the directrix (and in this case, the $x$ axis) that runs through the focus of the parabola. This line intersects the parabola at two points; one on either side of the focus. The distance between these points is the focal width (which is $4p$). So, the focal width can be defined simply as the distance between the two arms of the parabola when they have the same $y$ value as the focus.