In a plasma, the collision time τcoll is defined as the time in which the trajectory of a (charged) particle undergoes a change of direction of 90 degrees. Due to the long range of the Coulomb force, Coulomb interactions are typically small angle scattering events, so that this direction change typically requires a large number of interactions.
Consider a test particle with charge q, mass m, and velocity v colliding with bulk particles with charge q*, mass m*, and thermal velocity v*. Then the collision frequency ν = 1/τcoll is given by 
assuming v > v*, where mr = mm*/(m+m*) is the reduced mass and n* the bulk particle density. The factor ln Λ appears due to the accumulation of many small-angle collisions within a Debye sphere.
The dimensionless collisionality ν* is defined as 
See Connection length. The mean free path is estimated by the thermal velocity divided by the collision frequency, vth / ν.