A parabola is defined as the collection of points P in a plane, such that the distance from each point to a fixed point F equals its distance to a fixed line D. The point F is called as the focus of the parabola and the line D is its directrix.
The line through the focus F and perpendicular to the directrix D is called the axis of symmetry or simply as axis of the parabola. The point of intersection of the parabola with its axis of symmetry is called vertex V.
Note that the vertex is midway between the focus and the directrix. In fact, vertex is at the least distance from the focus. (See in figure given below.)