# What is the directrix of a parabola?

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.