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]