The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hypotenuse
    n 1: the side of a right triangle opposite the right angle

The Free On-line Dictionary of Computing (30 December 2018):

hypotenuse

    The side of a right-angled triangle opposite the
   right angle.

   (2004-02-15)