\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.