Proportionality constant between two quantities of the same dimension.
Sources:
ISO 31-0: 1992 (https://www.iso.org/iso/catalogue_detail.htm?csnumber=3621)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 973 (https://doi.org/10.1351/pac199668040957)