A theory relating quantities in equilibrium
and nonequilibrium statistical mechanics and microscopic
and macroscopic quantities.

The underlying principle
of the fluctuation-dissipation theorem is that a nonequilibrium
state may have been reached either as a result of
a random fluctuation or an external force and that
the evolution towards equilibrium is the same in both
cases.

It was first devised for electrical circuits with noise
in 1928 by H.Nyquist; a general theorem in statistical
mechanics was derived by H.B Callen and T.A Welton
in 1951.