spandrel

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The roughly triangular space between the left or right exterior curve of an arch and the rectangular framework surrounding it.
• noun The space between two arches and a horizontal molding or cornice above them.

from The Century Dictionary.

• noun In architecture, the triangular space comprehended between the outer curve or extrados of an arch, a horizontal line drawn through its apex, and a vertical line through its springing; also, the wall-space between the outer moldings of two arches and a horizontal line or string-course above them, or between these outer moldings and the intrados of another arch rising above and inclosing the two.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.
• noun Cant A narrow mat or passe partout for a picture.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun architecture The space (often triangular) between the outer curve of an arch (the extrados) and a straight-sided figure that bounds it; the space between two contiguous arches and a straight feature above them
• noun architecture The triangular space under a stair; the material that fills the space
• noun architecture A horizontal member between the windows of each storey of a tall building
• noun An oriental rug having a pattern of arches; the design in the corners of such a rug, especially in a prayer rug
• noun genetics A phenotypic characteristic that evolved as a side effect of a true adaptation

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an approximately triangular surface area between two adjacent arches and the horizontal plane above them