Quantities that characterize transport in a system. Examples of transport coefficients include electrical and thermal conductivity. One of the main purposes of nonequilibrium statistical mechanics is to calculate such coefficients from first principles. It is difficult to calculate transport coefficients exactly for systems with interactions between the particles in the system taken into account, and it is therefore necessary to use approximation techniques and/or model systems. A transport coefficient gives a measure for flow in a system. An inverse transport coefficient gives a measure of resistance to flow in a system. An example of an inverse transport coefficient is resistivity.