The latus rectum of a parabola. It is a third proportional to the abscissa and any ordinate, so that the square of the ordinate is always equal to the rectangle under the parameter and abscissa; but in the ellipsis and hyperbola it has a different proportion.

In conic sections, a third proportional to any diameter and its conjugate. In the parabola, a third proportional to any absciss and its ordinate.