Collision frequency

Average number of collisions per second of a charged particle of a given species with particles of another or the same species.

Source:Service de « Travaux publics et Services gouvernementaux Canada »

Formula

https://upload.wikimedia.org/math/c/e/b/cebc2f5432bc356917c8c071960285bd.png where NA is the Avogadro's constant, σAB the cross section, kB the Boltzmann constant and µAB the reduced mass of the reactants.