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.
Directrix. E.V. Shikin (originator), Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Directrix&oldid=11408