What is the directrix of a parabola?

The directrix of a parabola refers to a straight line that is drawn perpendicular to the axis of symmetry of a parabola. This line does not touch the parabola at any point. In addition, it is a horizontal line if the axis is vertical.

To draw a parabola, one has to start with the directrix and a point, known as focus that is off to one side. The parabola, which is the curve, is then formed from all points that are equidistant from the focus and the directrix. The axis of symmetry is a perpendicular line to the directrix that splits the parabola in the middle and passes through the focus.