It was established by Callen and Welton in 1951, and it may be formulated by noting that the linear susceptibility of an order parameter is given by minus beta times the time derivative of the correlation function.  
One of the terms in its name is the imaginary part of the generalized AC susceptibility. It was established by Callen and Welton in 1951, and it may be formulated by noting that the linear susceptibility of an order parameter is given by minus beta times the time derivative of the correlation function.  The susceptibility term characterizes an effect that could be associated with friction, and the result holds assuming the external interaction can be treated as a small perturbation.  On the other side of the equation, the correlation function characterizes the expectation value of statistical deviations of the perturbed quantity. FTP, what is this theorem relating two important quantities in statistical mechanics?
One of the terms in its name is the imaginary part of the generalized AC susceptibility.
fluctuation-dissipation theorem