Dispersion related to a third-order dependence of the phase change on the Frequency offset.
Third-order Dispersion results from the frequency dependence of the Group Delay dispersion. In the expansion of the Spectral Phase versus angular frequency offset, it is related to the third-order term.
In mode-locked lasers for pulse durations below roughly 30 femtoseconds, it is necessary to provide Dispersion Compensation not only for the group delay dispersion, but also for the third-order dispersion and possibly for even higher orders.
In many cases, the investigation of the effect of third order dispersion requires numerical pulse propagation modeling.