kinetic activity factor

https://doi.org/10.1351/goldbook.K03396
A factor involving activity coefficients that appears as a multiplier in the rate equation. For a bimolecular reaction the factor is $\frac{y_{\text{A}}\ y_{\text{B}}}{y_{\ddagger}}$ where y A, y B and y ‡ are the activity coefficients for the reactants A and B and the activated complex, respectively. For a unimolecular reaction the factor is $\frac{y_{\text{A}}}{y_{\ddagger}}$ and for a trimolecular reaction it is $\frac{y_{\text{A}}\ y_{\text{B}}\ y_{\text{C}}}{y_{\ddagger}}$
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 171 (https://doi.org/10.1351/pac199668010149)