The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]