• Directrix

    Noun

    Image:Eccentricity.svg|thumb|In this picture of an ellipse, a parabola, and an

    directrix

    (plural directrices)
    1. A female who directs; a directress.
    2. (geometry) A line used to define a curve or surface; especially a line, the distance from which a point on a conic has a constant ratio to that from the focus.
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