pendentive

noun Architecture.

1. any of several spandrels, in the form of spherical triangles, forming a transition between the circular plan of a dome and the polygonal plan of the supporting masonry.
2. any of several masonry devices, as squinches or trompes, for forming a transition between a circular or polygonal construction, as a dome or lantern, and supporting masonry of a different plan.

adjective

3. functioning as, or substituting for, a pendentive: pendentive corbeling.

noun

1. any of four triangular sections of vaulting with concave sides, positioned at a corner of a rectangular space to support a circular or polygonal dome

n.1727, from French pendentif (mid-16c.), from Latin pendentem (nominative pendens) “hanging,” present participle of pendere “to hang” (see pendent (adj.)).