Directrix

In mathematics, a directrix is a curve associated with a process generating a geometric object, such as:
 * Directrix (conic section)
 * Directrix (generatrix)
 * Directrix (rational normal scroll)

Other uses

 * Directrix is a spaceship in the Lensman series of novels by E. E. Smith.
 * Directrix is the name of an indie alternative band in Chicago.
 * Directrix - a feminine form of director in the context of grammatical gender.