= Aging in nonequilibrium systems
<Aging in nonequilibrium systems> means that the measured response or correlation depends on how long the system has been evolving since preparation, not only on the interval between measurements. A two-time correlation $C(t_w+t,t_w)$ depends separately on waiting time $t_w$ when time-translation invariance has not been established.
One probing protocol quenches a disordered magnet, waits different times and then applies a small field. Comparing the resulting responses distinguishes preparation history from a single <equilibrium> relaxation time. Analytical spin-glass models exhibit such nonequilibrium correlation and response behavior; simple scaling with $t/t_w$ is a model-dependent form, not the definition of aging.<Analytical solution of the off-equilibrium dynamics of a long-range spin-glass model by L. F. Cugliandolo and J. Kurchan>[Original nonequilibrium spin-glass dynamics and two-time response framework]{ref}
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