Directrix

A straight line lying in the plane of a conic section (an ellipse, a hyperbola or a parabola, cf. Conic sections) and having the property that the ratio between the distance from any point on the curve to a focus of the curve and the distance from that point to the straight line is a constant magnitude, by definition equal to the eccentricity.

Comments

An ellipse and a hyperbola have both two foci and two associated directrices; a parabola has one focus and one directrix. Cf. also the comments to Focus of a curve.