directrix

noun, plural di·rec·trix·es, di·rec·tri·ces [dih-rek-tri-seez, dahy-, dahy-rek-trahy-seez] /dɪˈrɛk trɪˌsiz, daɪ-, ˌdaɪ rɛkˈtraɪ siz/.

1. Geometry. a fixed line used in the description of a curve or surface.
2. Archaic. a directress.

noun

1. geometry a fixed reference line, situated on the convex side of a conic section, that is used when defining or calculating its eccentricity
2. a directress

1. A straight line used in generating a curve such as a parabola.