## stoichiometric number, $$\nu$$

https://doi.org/10.1351/goldbook.S06025
A chemical reaction of known stoichiometry can be written in general as: $a\text{A}\,+\,b\text{B}\,+\,...\rightarrow\,...\,+y\text{Y}\,+\,z\text{Z}$ For the reaction products Y and Z the numbers y and z are known as the stoichiometric numbers, ν Y and ν Z, for Y and Z respectively. For the reactants the stoichiometric numbers are the negatives of the numbers appearing in the equation; for example the stoichiometric number ν A for the reactant A is -a. In other words, the stoichiometric numbers are positive for products and negative for reactants.
Sources:
Green Book, 2nd ed., p. 42 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 187 (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 993 (https://doi.org/10.1351/pac199668040957)