SPANDREL
spandrel, spandril
(noun) an approximately triangular surface area between two adjacent arches and the horizontal plane above them
Source: WordNet® 3.1
Etymology
Noun
spandrel (plural spandrels)
(architecture) The space (often more or less 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.
Coordinate term: pendentive
(architecture) Horizontal member between the windows of successive storeys of a tall building.
(architecture) The triangular space under a stair; the material that fills the space.
An oriental rug having a pattern of arches; the design in the corners of such a rug, especially in a prayer rug.
(evolutionary theory) A phenotypic characteristic that evolved as a side effect of an adaptation in response to evolutionary pressure.
Source: Wiktionary
Span"drel, n. Etym: [From Span.]
1. (Arch.)
Definition: 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: Webster’s Unabridged Dictionary 1913 Edition