Source : Webster's Revised Unabridged Dictionary (1913)

Density \Den"si*ty\, n. [L. densitas; cf. F. densit['e].]
   1. The quality of being dense, close, or thick; compactness;
      -- opposed to rarity.

   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.

   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.

   3. (Photog.) Depth of shade. --Abney.

Source : WordNet®

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