measures how rapidly a perturbed system returns to equilibrium.
is an observable occurring in a perturbed Hamiltonian,
is an ensemble average and the dot above the A denotes the time derivative.
that are greater than the correlation time of the fluctuations of the observable the transport coefficient obeys a generalized Einstein relation: In general a transport coefficient is a tensor.
For strong gradients the transport equation typically has to be modified with higher order terms (and higher order Transport coefficients).