  Aims. The process of pitch-angle isotropization is important for many
applications ranging from diffusive shock acceleration to large-scale
cosmic-ray transport. Here, the basic analytical description is revisited on
the basis of recent simulation results. Methods. Both an analytical and a
numerical investigation were undertaken of the Fokker-Planck equation for
pitch-angle scattering. Additional test-particle simulations obtained with the
help of a Monte-Carlo code were used to verify the conclusions. Results. It is
shown that the usual definition of the pitch-angle Fokker-Planck coefficient
via the mean-square displacement is flawed. The reason can be traced back to
the assumption of homogeneity in time which does not hold for pitch-angle
scattering. Conclusions. Calculating the mean free path via the Fokker-Planck
coefficient has often proven to give an accurate description. For numerical
purposes, accordingly, it is the definition that has to be exchanged in favor
of the pitch-angle correlation function.
