pendentive (n.)

in architecture, "one of the triangular segments of the lower part of a hemispherical dome left by the penetration of the dome by two semicircular vaults intersection at right angles" (oh, hell, just look at a picture), 1727, from French pendentif (mid-16c.), from Latin pendentem (nominative pendens) "hanging," present participle of pendere "to hang" (from PIE root *(s)pen- "to draw, stretch, spin").