the ratio of the weight or mass of a given volume of a substance to that of an equal volume of another substance (water for liquids and solids, air or hydrogen for gases) used as a standard
See specific gravity in American Heritage Dictionary 4
noun
The ratio of the mass of a solid or liquid to the mass of an equal volume of distilled water at 4°C (39°F) or of a gas to an equal volume of air or hydrogen under prescribed conditions of temperature and pressure. Also called relative density.