The Collaborative International Dictionary of English v.0.48:

Turret \Tur"ret\, n. [OE. touret, OF. tourette, dim. of tour a
   tower, L. turris. See Tower.]
   [1913 Webster]
   1. (Arch.) A little tower, frequently a merely ornamental
      structure at one of the angles of a larger structure.
      [1913 Webster]

   2. (Anc. Mil.) A movable building, of a square form,
      consisting of ten or even twenty stories and sometimes one
      hundred and twenty cubits high, usually moved on wheels,
      and employed in approaching a fortified place, for
      carrying soldiers, engines, ladders, casting bridges, and
      other necessaries.
      [1913 Webster]

   3. (Mil.) A revolving tower constructed of thick iron plates,
      within which cannon are mounted. Turrets are used on
      vessels of war and on land.
      [1913 Webster]

   4. (Railroads) The elevated central portion of the roof of a
      passenger car. Its sides are pierced for light and
      ventilation.
      [1913 Webster]

   Turret clock, a large clock adapted for an elevated
      position, as in the tower of a church.

   Turret head (Mach.), a vertical cylindrical revolving tool
      holder for bringing different tools into action
      successively in a machine, as in a lathe.

   Turret lathe, a turning lathe having a turret head.

   Turret ship, an ironclad war vessel, with low sides, on
      which heavy guns are mounted within one or more iron
      turrets, which may be rotated, so that the guns may be
      made to bear in any required direction.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Turrethead \Tur"ret*head\
   an attachment fitted to a lathe or other machinery which
   holds a variety of tools which can act on the object being
   worked, and which are interchangeable by a pivoting motion,
   thus allowing efficient performance of multiple operations;
   -- also called a turret.
   [PJC]

WordNet (r) 3.0 (2006):

turret
    n 1: a small tower extending above a building
    2: a self-contained weapons platform housing guns and capable of
       rotation [syn: gun enclosure, gun turret, turret]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "turret":
   antenna tower, barbican, belfry, bell tower, campanile, colossus,
   column, cupola, derrick, dome, fire tower, lantern, lighthouse,
   martello, martello tower, mast, minaret, monument, obelisk,
   observation tower, pagoda, pilaster, pillar, pinnacle, pole, pylon,
   pyramid, shaft, skyscraper, spire, standpipe, steeple, stupa,
   television mast, tope, tour, tower, water tower, windmill tower