Separation of powers
Separation of powers, political theory developed by Montesquieu from his studies of the British constitution, arguing that the arbitrary exercise of government power should be avoided by dividing it between distinct departments: the executive, legislature, and judiciary. This was a basic principle of the Founding Fathers in producing the U.S. Constitution; legislative powers were vested in Congress, judicial powers in the Supreme Court and subsidiary courts, and executive powers in the president and his governmental machinery. Each branch was to have its functions, duties, and authority, and in theory no branch could encroach upon another. In practice there has always been a degree of necessary overlap. The legislature can oppose and impeach members of the executive, the president can veto legislation, and the Supreme Court can adjudicate the actions of the other branches; its members, in turn, are presidential appointees subject to congressional approval. In U.S. history one branch has always tended to dominate others for long periods, but this “checks and balances” ensures that power can and does shift between them.
See also: Montesquieu.