- noun the ratio of density of a liquid with reference to water, or of a gas with reference to air
- The ratio of the density of a given material or medium, to that of a standard material or medium. For example, the ratio of the density of a material to that of water at 4 °C at one atmosphere, or the ratio of the density of a gas to that of dry air at 0 °C at one atmosphere. Also called specific gravity.
- noun the ratio of the density of a substance to the density of a standard substance at the same temperature and pressure. For liquids and solids the standard substance is usually water, and for gases, it is air.