A precise measure of how strongly a molecule grips a receptor — a lower number means tighter binding.Thermodynamic measure of binding affinity, derived from equilibrium binding experiments. Unlike IC50 (which varies with assay conditions), Ki is a fundamental constant that describes the receptor-ligand interaction. Tight binding (low Ki) is necessary but not sufficient for therapeutic activity — a compound must bind, activate or block the receptor appropriately, and survive long enough in the body to do so.