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A straight line lying in the plane of a conic section (an ellipse, a hyperbola or a parabola, cf. [[Conic sections|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|eccentricity]].



====Comments====
An [[Ellipse|ellipse]] and a [[Hyperbola|hyperbola]] have both two foci and two associated directrices; a [[Parabola|parabola]] has one focus and one directrix. Cf. also the comments to [[Focus of a curve|Focus of a curve]].