- the ratio of the density of a substance to the density of a standard substance under specified conditions. For liquids and solids the standard is usually water at 4°C or some other specified temperature. For gases the standard is often air or hydrogen at the same temperature and pressure as the substanceSymbol: d See also specific gravity, vapour density
- The ratio of the density of one substance to that of a reference substance, typically water. See more at specific gravity.