The separation of powers is the constitutional principle that the functions of the state should be divided among distinct branches, typically the legislature, which makes law; the executive, which implements it; and the judiciary, which interprets and applies it.
Origins
The idea is most closely associated with Montesquieu's The Spirit of the Laws (1748), which argued that liberty cannot exist where the power to make laws, to execute them and to judge is concentrated in the same hands. The doctrine profoundly influenced the United States Constitution.
Checks and Balances
In practice, no system separates powers completely. Modern constitutions instead create checks and balances: the executive may veto or must sign legislation, the legislature controls the budget and scrutinises the executive, and the courts may review the legality and constitutionality of both legislative and executive acts.
The United Kingdom
The United Kingdom has a partial separation: ministers sit in Parliament, but the judiciary has been institutionally separate since the creation of the Supreme Court under the Constitutional Reform Act 2005.
In Indonesia
The amendments to the 1945 Constitution made between 1999 and 2002 strengthened the separation of powers. Legislative power rests principally with the House of Representatives (DPR), with the Regional Representative Council (DPD) and the People's Consultative Assembly (MPR) having defined roles; executive power rests with the President; and judicial power is exercised by the Supreme Court and the Constitutional Court, which are independent. The Audit Board (BPK) and the Judicial Commission provide additional checks.