What does the displacement from equilibrium represent in a relaxation experiment?

In a relaxation experiment, the displacement from equilibrium—often denoted as $\Delta X$—represents the difference between the instantaneous value and the equilibrium value of a specific property of the system.1

This property can vary depending on the system being studied; for example, it might refer to the kinetic energy of molecules behind a shock front or the concentration of a chemical reactant within a mixture.1

In practice, researchers often utilize small disturbances to ensure that the system's response can be described by a linear rate equation, where the rate at which the displacement disappears is approximately proportional to the magnitude of the displacement itself. Measuring how this displacement changes over time allows scientists to determine the relaxation time, which provides critical insights into atomic and molecular structures as well as the mechanisms and rates of chemical reactions.12