D defs.my
Entry 2 senses Webster, 1913

Directrix

· Di·rect·rix · IPA
01 n. A directress.
pl. Directrixes
  1. 1.
    A directress.[R.]
  2. 2.
    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.(Geom.)