What is the difference between relative density and density, and how do they relate to each other?

Density is a measure of how much mass is contained in a given volume of a substance, while relative density compares the density of a substance to the density of another substance, usually water. Relative density is also known as specific gravity. The relationship between relative density and density is that relative density is a comparison of densities, while density is an absolute measure of mass per unit volume.