rate of conversion, ξ .

https://dx.doi.org/10.1351/goldbook.R05147
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.
Source:
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
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 989