Equilibration
This refers to any process where drug “equilibrates”, or reaches an equilibrium, between two (or more) compartments, or two (or more) states (e.g. bound to a receptor or not bound to a receptor). A system in equilibrium indicates that drug movement occurs at an equal rate in both directions such that there is no net movement of drug. Equilibration does not necessarily mean that drug concentrations are equal on either side of the equilibrium. For example, at equilibrium, a lipid soluble drug may be present at 10 mg/litre in adipose tissue and at 0.1 mg/litre in the blood supply passing through the adipose tissue, but the rate of distribution of drug from blood into fat is equal to the rate of redistribution of drug from fat back into blood, and so the system is at equilibrium. In that example, the plasma concentration of drug at equilibrium would allow you to calculate the apparent volume of distribution at steady state (AVD SS) for the drug.
