The Collaborative International Dictionary of English v.0.48:

Subtend \Sub*tend"\, v. t. [imp. & p. p. Subtended; p. pr. &
   vb. n. Subtending.] [L. subtendere; sub under + tendere to
   stretch, extend. See Tend.]
   To extend under, or be opposed to; as, the line of a triangle
   which subtends the right angle; the chord subtends an arc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

subtend
    v 1: be opposite to; of angles and sides, in geometry [syn:
         subtend, delimit]