Cathetus \Cath"e*tus\, n.; pl. {catheti}. [L., fr. Gr. ? a perpendicular line, fr. ? let down, fr. ?. See {Catheter}.] (Geom.) One line or radius falling perpendicularly on another; as, the catheti of a right-angled triangle, that is, the two sides that include the right angle. --Barlow.