A constitution that combines monarchic, aristocratic and democratic elements so that each can check the others.
In more detail
Polybius, in Book 6 of his Histories, credited the Spartan legislator Lycurgus with the first such constitution and argued that Rome's consuls, Senate and people formed a working example. He held that simple constitutions decay quickly into their corrupt forms and that even a mixed one declines eventually. Modern scholars debate how well the model describes Roman practice.