Merriam-Webster Collegiate Dictionary - directrix

 noun  (plural ~es; also directrices)  Etymology: Medieval Latin, feminine of Late Latin director, from Latin dirigere  Date: 1622  1. archaic directress  2. a fixed curve with which a generatrix maintains a given relationship in generating a geometric figure; specifically a straight line the distance to which from any point of a conic section is in fixed ratio to the distance from the same point to a focus