Sometimes expressed in grams per cubic centimeter, it's mass divided by volume
[DOC] [TLE] Density - New World Encyclopedia [PAR] Aug 14, 2013 ...  is the object's density (measured in kilograms per cubic metre) ... often the 
density of water, but sometimes the air when comparing to gases): ... the same 
value as density of the material expressed in grams per cubic centimeter, ... 
Basically, density is defined as the mass of a sample divided by its volume.