Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density of a substance to the density of a given reference material. The relative density of solids and liquids is nearly always measured with respect to water at its densest ; for gases, the reference is air at room temperature. The term "relative density" is preferred modern use, for example in ISO, IUPAC, and NIST whereas the term "specific gravity" is gradually being abandoned.