Non-equilibrium thermodynamics is a branch of thermodynamics that deals with
physical systems that are not in thermodynamic equilibrium but can be
adequately described in terms of variables (non-equilibrium state variables)
that represent an extrapolation of the variables used to specify the system in
Almost all systems found in nature are not in thermodynamic equilibrium; for
they are changing or can be triggered to change over time, and are
continuously and discontinuously subject to flux of matter and energy to and
from other systems.
One fundamental difference between equilibrium thermodynamics and
non-equilibrium thermodynamics lies in the behaviour of inhomogeneous
systems, which require for their study knowledge of rates of reaction which
are not considered in equilibrium thermodynamics.
Another fundamental and very important difference is the difficulty or
impossibility in defining entropy at an instant of time in macroscopic terms
for systems not in thermodynamic equilibrium.