Noun

1. 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
2. The triangular space under a stair; the material that fills the space
3. A horizontal member between the windows of each storey of a tall building
4. An oriental rug having a pattern of arches; the design in the corners of such a rug, especially in a prayer rug
5. A phenotypic characteristic that evolved as a side effect of a true adaptation