Mixed constitution

Mixed government (or a mixed constitution) is a form of government that combines elements of democracy, , and monarchy, making impossible their respective degenerations (conceived as anarchy, oligarchy and tyranny). The idea was popularized during classical antiquity in order to describe the stability, the innovation and the success of the republic as a form of government developed under the Roman constitution.

Unlike classical democracy, aristocracy or monarchy, under a mixed government rulers are elected by citizens rather than acquiring their positions by inheritance or sortition (at the Greco-Roman time, sortition was conventionally regarded as the principal characteristic of classical democracy).

The concept of a mixed government was studied during the Renaissance and the Age of Reason, by Machiavelli, Vico, Kant, Hobbes and others. It was, and is, a very important theory among supporters of republicanism. Various schools have described modern polities, such as the EU and the US, as possessing mixed constitutions.

Plato in his book The Republic divided governments into five basic types (Four being existing forms and one being Plato's ideal form, which exists "only in speech"):

Plato found flaws with all existing forms of government and thus concluded that aristocracy, which emphasizes virtue and wisdom, is the purest form of government. Aristotle largely embraced Plato's ideas and in his Politics three types (excluding timocracy) are discussed in detail. Aristotle considers constitutional government (a combination of oligarchy and democracy under law) the ideal form of government, but he observes that none of the three are healthy and that states will cycle between the three forms in an abrupt and chaotic process known as the kyklos or anacyclosis. In his Politics he lists a number of theories of how to create a stable government. One of these options is creating a government that is a mix of all three forms of government.

...
Wikipedia