Specific Gravity

The dimensionless ratio of a substance's mass density to the mass density of a specified reference substance under stated conditions (commonly water for liquids and solids); it indicates relative heaviness without units and requires the reference substance and its temperature/pressure to be specified.

The dimensionless ratio of the density of a material (mass per unit volume) to the density of water under specified reference conditions; used as a comparative measure of material density independent of units.