Given a point F (the focus) and a line (the directrix) that lie in a plane, the locus of points in that plane that are equidistant from F and is a parabola.

One form of the equation of a parabola is , where the focal parameter is the distance from F to .

This Demonstration illustrates the reflection property of parabolas. Imagine that the parabola is a mirror and that there is a light source at the focus. Then all the reflected rays are parallel to the axis of symmetry of the parabola.