A factor involving activity coefficients that appears as a multiplier in the rate equation. For a bimolecular reaction the factor is $$\frac{y_{\text{A}}{y}_{\text{B}}}{y_{‡}}$$ where $y_{\text{A}}$, $y_{\text{B}}$ and $y_{‡}$ are the activity coefficients for the reactants A and B and the activated complex, respectively. For a unimolecular reaction the factor is $$\frac{y_{\text{A}}}{y_{‡}}$$ and for a trimolecular reaction it is $$\frac{y_{\text{A}}{y}_{\text{B}}{y}_{\text{C}}}{y_{‡}}$$