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/.
Examples from the Web for directrix
The surface formed by revolving the catenary about its directrix is named the alysseide.
If a parabola roll on another parabola, their vertices coinciding, the focus of the first traces out the directrix of the second.The Romance of Mathematics|P. Hampson
The centre of a circle is its focus, and its directrix has gone to infinity, having no special direction.
Similarly, in an hyperbola a vertex is nearer 704 to the directrix than to the focus.
If the directrix, focus and eccentricity be given, we may employ the general method for constructing a conic.