rate of conversion, $$\dot{\xi}$$

https://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: $\overset{\text{.}}{\xi }=\frac{\mathrm{d}\xi }{\mathrm{d}t}$ In view of the definition of extent of reaction it follows that with reference to any species in a reaction showing time-independent stoichiometry $\overset{\text{.}}{\xi }=\frac{\mathrm{d}\xi }{\mathrm{d}t}=\frac{1}{\nu _{i}}\ \frac{\mathrm{d}n_{i}}{\mathrm{d}t}$ where n i is the amount of the species at any time and ν i is its stoichiometric coefficient.
Sources:
Green Book, 2nd ed., p. 55 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1992, 64, 1569. 'Quantities and units for metabolic processes as a function of time (IUPAC Recommendations 1992)' on page 1573 (https://doi.org/10.1351/pac199264101569)
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 180 (https://doi.org/10.1351/pac199668010149)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 989 (https://doi.org/10.1351/pac199668040957)