Pendentive
Pendentive (pen*dent"iv) , noun
[French pendentif, from Latin pendere to hang.]
(a)
(Architecture) The portion of a vault by means of which the square space in the middle of a building is brought to an octagon or circle to receive a cupola.
(b)
(Architecture) The part of a groined vault which is supported by, and springs from, one pier or corbel.