What does "density" mean?

Density: (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 0º Centigrade and a pressure of 760 millimeters. For liquids and solids the standard is water at a temperature of 4º 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.