Spandrel
The triangular space between the outer curve of an arch and the rectangle formed by the mouldings enclosing it, frequently filled in with ornamental work.
Any similar space between an arch and a straight-sided figure bounding it.
The space included between the shoulders of two contiguous arches and the moulding or string-course above them.
And hence by association:
The support of a set of steps; the material with which the space between a stair and the floor is filled in.
On oriental patterned rugs or carpets: one of the spaces between the central field and the border, or between an arch motif and its frame.
By extension spandrel is sometimes used to refer to any ornamental bracket.
Additionally in evolutionary biology, a spandrel is a characteristic that is a by-product of the evolution of some other characteristic, rather than a direct product of adaptive selection. In other words, an evolutionary bracket or ornament.
The OED records the earliest use as being in 1477-8 and the word is thought to derive from a diminutive of the Ancient French spaundre, which is itself of doubtful origin.