stoichiometric number, ν

A chemical reaction of known stoichiometry can be written in general as:
a A + b B + ... → ... + y Y + z 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.
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 187
Green Book, 2nd ed., p. 42
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 993
See also:
PAC, 1996, 68, 1167 (Pesticides report 36. Glossary of terms relating to pesticides (IUPAC Recommendations 1996)) on page 1168