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, 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 Obliteration, a right line drawn perpendicular to the spectrum, in the point of incidence or reflection.

Architecture, a vertical line supposed to pass through the centre of a cylindrical body.