Specific gravity is a comparison of the densities of two substances, one of which is usually pure water. Specific gravity is measured as a ratio. It is a dimensionless quantity, which means that it has no units of measurement associated with it. When the specific gravity of a substance is measured, it is usually measured in air rather than in a vacuum. (A vacuum is an environment in which no matter, including air, exists.) The term specific gravity is often abbreviated as sp gr.
Salem Press Encyclopedia of Science, 2019. 2p.