Separation of Powers
The constitutional doctrine that allocates the powers of national government among three branches: the legislative, which is empowered to make laws; the executive, which is required to carry out the laws; and the judicial, whose job it is to interpret and adjudicate (hear and decide) legal disputes. Some powers belong exclusively to a single branch; others are shared among the branches.
For example, the President is commander in chief of the Armed Forces, but Congress has the power to declare war and to raise and fund the armies. The framers believed that this separation of powers would ensure that no one person or group of persons would be able to create, administer and enforce the laws at the same time and thereby become too powerful. Each branch would be a check on the power of the other two branches.
The Constitution also requires that no one person serve in more than one branch simultaneously. During the founding era of the United States, James Madison expressed the importance of separated powers in a constitutional government. In the 47th paper of "The Federalist," Madison wrote, "The accumulation of all powers, legislative, executive and judiciary, in the hands of one, a few, or many, and whether hereditary, self-appointed, or elected, may justly be pronounced the very definition of tyranny."
Defenders of separated and shared powers emphasize the importance of deliberate decision-making in support of their system of constitutional democracy. They believe that the compromises necessary to achieve agreement among different groups empowered with checks on the actions of the other groups result in a government that cannot act recklessly.
Justice Louis D. Brandeis of the U.S. Supreme Court nicely summed up the justification for separated and shared powers in the Constitution. In his dissenting opinion in the 1926 case Myers v. United States, Justice Brandeis wrote, “The doctrine of the separation of powers was adopted by the Convention of 1787, not to promote efficiency but to preclude the exercise of arbitrary power. The purpose was not to avoid friction but, by means of the inevitable friction incident to the distribution of the governmental powers among three departments, to save the people from autocracy.”
Portions of this definition by John Patrick, Understanding Democracy, A Hip Pocket Guide (Oxford University Press)