Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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