Pendentive

is an architectural term used to designate the portion of a groined ceiling supported by one pillar or impost, and bounded by the apex of the longitudinal and transverse vaults; in Gothic ceilings of this kind the ribs of the vaults descend from the apex to the impost of each pendentive, where they become united. It also denotes the portion of a domical vault which descends into the corner of an angular building when a ceiling of this description is placed over a straight-sided area; pendentives of this kind are common in Byzantine architecture, but not in Gothic.