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 yA yBy where yA, yB and y are the activity coefficients for the reactants A and B and the activated complex, respectively. For a unimolecular reaction the factor is yAy and for a trimolecular reaction it is yA yB yCy
Source:
PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 171 [Terms] [Paper]