The Collaborative International Dictionary of English v.0.48:

Density \Den"si*ty\ (d[e^]n"s[i^]*t[y^]), n. [L. densitas; cf.
   F. densit['e].]
   1. The quality of being dense, close, or thick; compactness;
      -- opposed to rarity.
      [1913 Webster]

   2. (Physics) The ratio of mass, or quantity of matter, to
      bulk or volume, esp. as compared with the mass and volume
      of a portion of some substance used as a standard.
      [1913 Webster]

   Note: For gases the standard substance is hydrogen, at a
         temperature of 0[deg] Centigrade and a pressure of 760
         millimeters. For liquids and solids the standard is
         water at a temperature of 4[deg] Centigrade. The
         density of solids and liquids is usually called
         specific gravity, and the same is true of gases when
         referred to air as a standard.
         [1913 Webster]

   3. (Photog.) Depth of shade. --Abney.
      [1913 Webster]

WordNet (r) 3.0 (2006):

density
    n 1: the amount per unit size [syn: density, denseness]
    2: the spatial property of being crowded together [syn:
       concentration, density, denseness, tightness,
       compactness] [ant: dispersion, distribution]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "density":
   asininity, blockishness, body, boobishness, boorishness, bovinity,
   callosity, callousness, cloddishness, concreteness, crassness,
   crudeness, denseness, dim-wittedness, dimness, doltishness,
   dull-wittedness, dullardism, dullness, dumbness, duncery,
   durability, durity, firmness, flintiness, grossness, hardness,
   hardness of heart, hebetude, impenetrability, induration, lethargy,
   loutishness, lumpishness, mass, materiality, ninnyism, oafdom,
   oafishness, obduracy, obtuseness, opacity, palpability,
   ponderability, resistance, restiveness, simpletonianism, slowness,
   sluggishness, solidity, sottishness, soundness, stability,
   steadiness, steeliness, stolidity, stoniness, stoutness, strength,
   stupidity, sturdiness, substance, substantiality, substantialness,
   tangibility, thick-headedness, thick-wittedness, toughness,
   unteachability, wrongheadedness, yokelism