Directrix of a conic
= Directrix of a conic
A <directrix of a conic> is the fixed line in its focus-directrix description: distance to the <focus of a conic> is a constant multiple of distance to this line. For a <parabola> the multiple is one. The <directrix of a conic> belongs to the geometry of the <conic section>, rather than being a tangent or an asymptote.