rate of conversion, ξ .

The rate of conversion for a reaction occurring in a closed system is defined as the time derivative of the extent of reaction:
ξ . = d ξ d t
In view of the definition of extent of reaction it follows that with reference to any species in a reaction showing time-independent stoichiometry
ξ . = d ξ d t = 1 ν i d n i d t
where n i is the amount of the species at any time and ν i is its stoichiometriccoefficient.
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 180
PAC, 1992, 64, 1569 (Quantities and units for metabolic processes as a function of time (IUPAC Recommendations 1992)) on page 1573
Green Book, 2nd ed., p. 55
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 989