Source : Webster's Revised Unabridged Dictionary (1913)

Hypotenuse \Hy*pot"e*nuse\, Hypothenuse \Hy*poth"e*nuse\, n. [L.
   hypotenusa, Gr. ?, prob., subtending (sc. ?), fr. ? to
   stretch under, subtend; ? under + ? to stretch. See
   {Subtend}.] (Geom.)
   The side of a right-angled triangle that is opposite to the
   right angle.

Source : WordNet®

hypotenuse
     n : the side of a right triangle opposite the right angle