## Wikipedia - Gravità specifica (it) relative density, $$d$$

https://doi.org/10.1351/goldbook.R05262
Ratio of density to a reference density, usually the density of water at $$4\ ^{\,\unicode{x26ac}}\text{C}$$; in the older literature this is called @S05799@.
Sources:
Green Book, 2nd ed., p. 12 [Terms] [Book]
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2210 [Terms] [Paper]