Resonant and Non Resonant Susceptibility

The imaginary part of the $$\chi ^3$$ susceptibility can be split into the resonant and non resonant contributions.
The resonant is the contribution from satisfying a resonant vibrational state/mode, but what contributes to the non resonant term?

The imaginary part of the $$\chi ^3$$ susceptibility can be split into the resonant and non resonant contributions.