The Collaborative International Dictionary of English v.0.48:

Generatrix \Gen`er*a"trix\, n.; pl. L. Generatrices, E.
   Generatrixes. [L.] (Geom.)
   That which generates; the point, or the mathematical
   magnitude, which, by its motion, generates another magnitude,
   as a line, surface, or solid; -- called also describent.
   Generic

The Collaborative International Dictionary of English v.0.48:

Describent \De*scrib"ent\, n. [L. describens, p. pr. of
   describere.] (Geom.)
   Same as Generatrix.
   [1913 Webster]