Average number of collisions experienced by a molecule A with other molecules A (or B) in a time interval divided by that time interval.
Green Book, 2nd ed., p. 56 [Terms] [Book]
See also:
PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 159 [Terms] [Paper]