A factor introduced into simple versions of the @C01170@ of reactions to take care of the fact that the @R05176@ depends on the certain mutual orientations of the reactant molecules.
See also:
collision frequency
PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 187 [Terms] [Paper]