specific gravity

relative weight of matter in contrast to the weight of the water in the volume

Relative density is a dimensionless ratio of the densities of two materials. The term specific gravity is similar, except that the reference material is water. A relative density can help quantify the buoyancy between two materials, or determine the density of one "unknown" material using the "known" density of another material. Mathematically, relative density is expressed as:
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Noun
1. the density of a substance relative to the density of water
(hypernym) relative density

The ratio of the weight of a particular volume of a given substance to the weight of an equal volume of pure water.

Compare with density .The mass of a unit volume of a substance relative to the mass of a unit volume of water. Temperature must be specified when reporting specific gravities, since the density of the substance and of water change with temperature. Specific gravities are often reported relative to water at 4°C; at that temperature, water has a density of 1.00000 g/mL and the specific gravity of a substance is equal to its density in g/mL.