Merriam-Webster Collegiate Dictionary - vestibule

 noun  Etymology: Latin vestibulum forecourt  Date: 1726  1.  a. a passage, hall, or room between the outer door and the interior of a building ; lobby  b. an enclosed entrance at the end of a railway passenger car  2. any of various bodily cavities especially when serving as or resembling an entrance to some other cavity or space: as  a. the central cavity of the bony labyrinth of the inner ear or the parts (as the saccule and utricle) of the membranous labyrinth that it contains  b. the part of the left ventricle below the aortic orifice  c. the space between the labia minora containing the orifice of the urethra  d. the part of the mouth cavity outside the teeth and gums  3. a course that offers access (as to something new)  • ~d adjective