Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material.
Applicable units are those of quantitykind:RelativeMassDensity
http://en.wikipedia.org/wiki/Relative_density
Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material.
$d = \frac{\rho}{\rho_0}$, where $\rho$ is mass density of a substance and $\rho_0$ is the mass density of a reference substance under conditions that should be specified for both substances.
http://www.iso.org/iso/catalogue_detail?csnumber=31889