Separation of powers refers to a system of government in which power is not centralized in one person or agency. In contrast to a dictatorship, the power to make and enforce various laws is spread among different groups or individuals. This form of government is used in the United States, as well as in the United Kingdom and several other democratic countries. Any government with a bicameral or tricameral legislature, for example, is practicing the principles of separation of powers.
Separation of powers in the United States is guaranteed by the Constitution. Following the American Revolution and breaking free from British rule, the founding fathers were concerned about the creation of one central government. As such, the Continental Congress that drafted the Constitution aimed to limit the power of the central government and ensure a system wherein power was distributed among different branches of government, as well as between the federal and state governments. Thus, they wished to ensure separation of powers so no abuses could take place.
The Constitution of the United States thus institutionalized separation of powers by creating three separate and distinct branches of government. The legislature has the power to make and propose laws, subject to presidential veto and judicial review by the Supreme Court. The president has the power to suggest legislation, but the Congress must approve it and the court can also review it for constitutionality. The Supreme Court is allowed to create case law and interpret case law, but the legislature can then pass a law changing the court's rules or interpretations. As such, each branch is subject to supervision and review by the other branches to ensure no power is abused.
Article III Section II of the Constitution also ensured that there would be shared power between the federal government and the state governments. This section of the Constitution mandated that all powers not expressly enumerated as being granted to the federal government were reserved for the states and the people. As such, if the Constitution didn't tell the federal government it had the authority to regulate a certain area, the state government has the exclusive jurisdiction to make and pass laws relating to that subject. Property and divorce laws, for example, are largely made at the state level since the state has the greatest vested interest in making and enforcing such laws.