The Collaborative International Dictionary of English v.0.48:

Latus rectum \La"tus rec"tum\n. [L., the right side.] (Conic
   Sections)
   The line drawn through a focus of a conic section parallel to
   the directrix and terminated both ways by the curve. It is
   the parameter of the principal axis. See Focus, and
   Parameter.
   [1913 Webster]