spandrel

(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.
(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.