Question

What is the directrix of a parabola?

Answer 1

The directrix of the parabola is a straight line that, together with the focus (a point), is used in one of the most common definition of parabolas.
In fact, a parabola can be defined as *the locus of points `P` such that the distance to the focus `F` equals the distance to the directrix `d`.

The directrix has the property of being always perpendicular to the axis of symmetry of the parabola.