1. a fixed line used in the definition of a conic section (such as a parabola, ellipse, or hyperbola) along with a focus point
Usage: geometry; mathematics

Examples

• The parabola is defined as the set of all points equidistant from the focus and the directrix.
• In geometry class, we learned how to identify the directrix of a given parabola.
• The directrix is perpendicular to the axis of symmetry of the parabola.
• To graph the conic section, first locate the focus and draw the directrix.
• The distance from any point on the ellipse to the focus divided by its distance to the directrix equals the eccentricity.
• Students often confuse the directrix with the axis of symmetry when studying parabolas.