Consortium imperii
Consortium imperii is a Latin term dating from the Roman Dominate, denoting the sharing of imperial authority between two or more emperors, each hence designated as consors imperii, i.e., "partner in (exercising) imperium", either as formal equals or in subordination, in which case the junior was often the senior's designated heir—not necessarily the natural one—and successor. In theory, this arrangement served to share the burden of government and ensure smooth succession, as rivalry at these moments was a major threat to the stability of the Empire; however, in practice the net result was often more civil war.