Source : Webster's Revised Unabridged Dictionary (1913)

Spandrel \Span"drel\, n. [From {Span}.]
   1. (Arch.) The irregular triangular space between the curve
      of an arch and the inclosing right angle; or the space
      between the outer moldings of two contiguous arches and a
      horizontal line above them, or another arch above and
      inclosing them.

   2. A narrow mat or passe partout for a picture. [Cant]

Source : WordNet®

spandrel
     n : an approximately triangular surface area between two
         adjacent arches and the horizontal plane above them [syn:
          {spandril}]