What is the “focal width” of a parabola?
Imagine a regular x2 parabola. It is facing up, and the vertex is at (0,0)(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.