spandrel

noun

1. 1.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.
2. 2.Horizontal member between the windows of successive storeys of a tall building.
3. 3.The triangular space under a stair; the material that fills the space.
4. 4.An oriental rug having a pattern of arches; the design in the corners of such a rug, especially in a prayer rug.
5. 5.A phenotypic characteristic that evolved as a side effect of an adaptation in response to evolutionary pressure.