Density is mass per unit volume, and the SI (or Systeme International) unit is kilogram per cubic meter, abbreviated as kg/m^3. This means that finding the density of a particular object requires dividing its mass by its volume.
Density defines how tightly packed together the matter in an object is. Density is important in determining the interaction of various materials with one another. For instance, in order to float in a particular medium, an object's density must be lower than the density of the medium. Density was first explained by Archimedes, a Greek scientist who lived from 287 to 212 B.C.E. Knowing the density of a certain object and either its mass or its volume allows one to calculate the other. For example, mass equals density times volume, and volume equals mass divided by density.