Archit. one of the triangular pieces of vaulting springing from the corners of a rectangular area, serving to support a rounded or polygonal dome: usually supported by a single pier
See pendentive in American Heritage Dictionary 4
(pĕn-dĕnˈtĭv)
noun Architecture
A triangular section of vaulting between the rim of a dome and each adjacent pair of the arches that support it.