A transport coefficient  measures how rapidly a perturbed system returns to equilibrium.
The transport coefficients occur in transport phenomenon with transport laws 
where:
:  is a flux of the property 
: the transport coefficient  of this property 
: , the gradient force which acts on the property .
Transport coefficients can be expressed via a Green–Kubo relation:
where  is an observable occurring in a perturbed Hamiltonian,  is an ensemble average and the dot above the A denotes the time derivative.
For times  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.