Cathetus

from The Century Dictionary.

• noun In geometry, a line falling perpendicularly on another line or a surface, as the two sides of a right-angled triangle.
• noun In architecture: A perpendicular line supposed to pass through the middle of a cylindrical body. The axis or middle line of the Ionic volute.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Geom.) One line or radius falling perpendicularly on another.