Quotient of frictional force and normal force, for a sliding body. Formerly called frictional coefficient or coefficient of friction.
Source:
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 974 (https://doi.org/10.1351/pac199668040957)