relative density, d

Ratio of density to a reference density, usually the density of water at 4 °C; in the older literature this is called specific gravity.
Green Book, 2nd ed., p. 12
PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2210
See also:
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 990