directrix

n. fixed line associated with a parabola or another curve (Geometry); directress

n. manager; stage manager

In mathematics, a conic section (or just conic) is a curve that can be formed by intersecting a cone (more precisely, a right circular conical surface) with a plane. The conic sections were named and studied as long ago as 200 BC, when Apollonius of Perga undertook a systematic study of their properties.
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(n.)
A straight line so situated with respect to a conic section that the distance of any point of the curve from it has a constant ratio to the distance of the same point from the focus.
  

(n.)
A line along which a point in another line moves, or which in any way governs the motion of the point and determines the position of the curve generated by it; the line along which the generatrix moves in generating a surface.
  

(n.)
A directress.
  

directrix /di'rektriks/
danh từ, số nhiều directrices (toán học) đường chuẩn (như) directress
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DIRETTRICE. DIRIGENTE [DONNA]