The Collaborative International Dictionary of English v.0.48:

Millstone \Mill"stone`\, n.
   One of two circular stones used for grinding grain or other
   substance in a mill[1].
   [1913 Webster]

         No man shall take the nether or the upper millstone to
         pledge.                                  --Deut. xxiv.
                                                  6.
   [1913 Webster]

   Note: The cellular siliceous rock called buhrstone is usually
         employed for millstones; also, some kinds of lava, as
         that Niedermendig, or other firm rock with rough
         texture. The surface of a millstone has usually a
         series of radial grooves in which the powdered material
         collects.
         [1913 Webster]

   Millstone girt (Geol.), a hard and coarse, gritty
      sandstone, dividing the Carboniferous from the
      Subcarboniferous strata. See Farewell rock, under
      Farewell, a., and Chart of Geology.

   To see into a millstone or To see through a millstone, to
      see into or through a difficult matter. (Colloq.)
      [1913 Webster]

WordNet (r) 3.0 (2006):

millstone
    n 1: (figurative) something that hinders or handicaps; "she was
         an albatross around his neck" [syn: albatross,
         millstone]
    2: any load that is difficult to carry
    3: one of a pair of heavy flat disk-shaped stones that are
       rotated against one another to grind the grain

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "millstone":
   bale, burden, burdening, burthen, cargo, charge, charging,
   cheese grater, comminutor, crusher, cumber, cumbrance, deadweight,
   drag, duty, encumbrance, freight, granulator, grater, grinder,
   handicap, incubus, incumbency, kominuter, lading, levigator, load,
   loading, masher, mill, mortar and pestle, nutmeg grater, onus,
   oppression, overload, overtaxing, overweighting, pestle, pressure,
   pulverizer, quern, quernstone, rock crusher, roller, saddling,
   shredder, steamroller, superincumbency, surcharge, task, tax,
   taxing, triturator, weight



U.S. Gazetteer Places (2000):

Millstone, NJ -- U.S. borough in New Jersey
   Population (2000):    410
   Housing Units (2000): 173
   Land area (2000):     0.749397 sq. miles (1.940929 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.749397 sq. miles (1.940929 sq. km)
   FIPS code:            46590
   Located within:       New Jersey (NJ), FIPS 34
   Location:             40.499453 N, 74.590875 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Millstone, NJ
    Millstone