n. a fixed line, on the convex side of a conic section, in terms of which, together with the focus and the eccentricity, the locus of points that constitute the conic is defined. For example, in the ellipse is defined as the locus of points whose distance from the focus is in some fixed proportion to their perpendicular distance from the line XYZ; P, Q, and R are three such points. XYZ is the directrix of the ellipse. See conic.