Proportionality constant between two quantities of the same dimension.
ISO 31-0: 1992
(Quantities and Units - Part 0: General Principles, Units and Symbols.)
PAC, 1996, 68, 957
(Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations
on page 973
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook