The Collaborative International Dictionary of English v.0.48:

Gallery \Gal"ler*y\, n.; pl. Galleries. [F. galerie, It.
   galleria, fr. LL. galeria gallery, perh. orig., a festal
   hall, banquetting hall; cf. OF. galerie a rejoicing, fr.
   galer to rejoice. Cf. Gallant, a.]
   1. A long and narrow corridor, or place for walking; a
      connecting passageway, as between one room and another;
      also, a long hole or passage excavated by a boring or
      burrowing animal.
      [1913 Webster]

   2. A room for the exhibition of works of art; as, a picture
      gallery; hence, also, a large or important collection of
      paintings, sculptures, etc.
      [1913 Webster]

   3. A long and narrow platform attached to one or more sides
      of public hall or the interior of a church, and supported
      by brackets or columns; -- sometimes intended to be
      occupied by musicians or spectators, sometimes designed
      merely to increase the capacity of the hall.
      [1913 Webster]

   4. (Naut.) A frame, like a balcony, projecting from the stern
      or quarter of a ship, and hence called stern gallery or
      quarter gallery, -- seldom found in vessels built since
      1850.
      [1913 Webster]

   5. (Fort.) Any communication which is covered overhead as
      well as at the sides. When prepared for defense, it is a
      defensive gallery.
      [1913 Webster]

   6. (Mining) A working drift or level.
      [1913 Webster]

   Whispering gallery. See under Whispering.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Whispering \Whis"per*ing\,
   a. & n. from Whisper. v. t.
   [1913 Webster]

   Whispering gallery, or Whispering dome, one of such a
      form that sounds produced in certain parts of it are
      concentrated by reflection from the walls to another part,
      so that whispers or feeble sounds are audible at a much
      greater distance than under ordinary circumstances. The
      dome of the U. S. capitol building is one example.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

whispering gallery
    n 1: a space beneath a dome or arch in which sounds produced at
         certain points are clearly audible at certain distant
         points [syn: whispering gallery, whispering dome]