The Collaborative International Dictionary of English v.0.48:

Vestibule \Ves"ti*bule\, n. [L. vestibulum, of uncertain origin:
   cf. F. vestibule.]
   The porch or entrance into a house; a hall or antechamber
   next the entrance; a lobby; a porch; a hall.
   [1913 Webster]

   Vestibule of the ear. (Anat.) See under Ear.

   Vestibule of the vulva (Anat.), a triangular space between
      the nymphae, in which the orifice of the urethra is
      situated.

   Vestibule train (Railroads), a train of passenger cars
      having the space between the end doors of adjacent cars
      inclosed, so as to admit of leaving the doors open to
      provide for intercommunication between all the cars.
      [1913 Webster]

   Syn: Hall; passage.

   Usage: Vestibule, Hall, Passage. A vestibule is a small
          apartment within the doors of a building. A hall is
          the first large apartment beyond the vestibule, and,
          in the United States, is often long and narrow,
          serving as a passage to the several apartments. In
          England, the hall is generally square or oblong, and a
          long, narrow space of entrance is called a passage,
          not a hall, as in America. Vestibule is often used in
          a figurative sense to denote a place of entrance. "The
          citizens of Rome placed the images of their ancestors
          in the vestibules of their houses." --Bolingbroke
          [1913 Webster]

WordNet (r) 3.0 (2006):

vestibule of the ear
    n 1: the central cavity of the bony labyrinth of the ear