collision frequency, z A A

https://dx.doi.org/10.1351/goldbook.C01166
Average number of collisions experienced by a molecule A with other molecules A (or B) in a time interval divided by that time interval.
Source:
See also:
PAC, 1996, 68, 149 (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 159