A nonlinear interaction of Light in a medium with an instantaneous response, related to the nonlinear electronic polarization.

The Kerr effect is a nonlinear optical effect occurring in crystals and glasses, but also in other media such as gases. Its physical origin is a nonlinear Polarization generated in the medium, which itself modifies the propagation of the light. The Kerr effect is the effect of an instantaneously occurring nonlinear response, which can be described as modifying the refractive index.