Ratio of density to a reference density, usually the density of water at 4°C; in the older literature this is called specific gravity.
Sources:
Green Book, 2nd ed., p. 12 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2210 (https://doi.org/10.1351/pac199062112167)
See also:
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 990 (https://doi.org/10.1351/pac199668040957)