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@.
Green Book, 2nd ed., p. 12 [Terms] [Book]
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2210 [Terms] [Paper]
See also:
PAC, 1996, 68, 957. (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 990 [Terms] [Paper]