The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. A narrow mat or passe partout for a picture. [Cant]
      [1913 Webster]

WordNet (r) 3.0 (2006):

spandrel
    n 1: an approximately triangular surface area between two
         adjacent arches and the horizontal plane above them [syn:
         spandrel, spandril]