CATHETUS noun 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 catoptries, is 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 obliquation, a right line drawn perpendicular to the speculum, in the point of incidence or reflection.

In architecture, a cathetus is a perpendicular line, supposed to pass through the middle of a cylindrical body.