Velocity of sedimentation divided by the acceleration of the force field (gravitation or centrifugation).
Sources:
Green Book, 2nd ed., p. 63 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 992 (https://doi.org/10.1351/pac199668040957)
See also:
Purple Book, 1st ed., p. 62 (http://old.iupac.org/publications/books/author/metanomski.html)
PAC, 1994, 66, 897. 'Quantities and units for centrifugation in the clinical laboratory (IUPAC Recommendations 1994)' on page 904 (https://doi.org/10.1351/pac199466040897)