The Collaborative International Dictionary of English v.0.48:

Pendentive \Pen*den"tive\ (p[e^]n*d[e^]nt"[i^]v), 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.
          [1913 Webster]