Source : Webster's Revised Unabridged Dictionary (1913)

Pendentive \Pen*den"tive\, n. [F. pendentif, fr. L. pendere to
   hang.] (Arch.)
      (a) 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) The part of a groined vault which is supported by, and
          springs from, one pier or corbel.