CATHETUS, in Geometry, a line or radius falling perpendicularly on another line or surface; as the two sides of a right-angled triangle.
CATHETUS of Incidence, in Catoptrics, a right line drawn from a point of the object, perpendicular to the reflecting line.
CATHETUS of Reflection, or of the Eye, a right line drawn from the eye, perpendicular to the reflecting plane.
CATHETUS of Obligation, a right line drawn perpendicularly to the speculum, in the point of incidence or reflection.