Separation of Powers
The constitutional principle that the legislative, executive, and judicial functions of government should be exercised by separate institutions, so each can check the others.
Origin
The idea goes back to Aristotle but was formalized by Montesquieu in The Spirit of the Laws (1748), who argued that political liberty requires power to balance power. The US Constitution is the cleanest implementation: Congress legislates, the President executes, the Supreme Court adjudicates, and each can block the others ('checks and balances'). Parliamentary systems (UK, Germany, India) fuse executive and legislature but still keep an independent judiciary. Authoritarian regimes typically preserve the names of the three branches while gutting any real independence between them.
Modern usage
Standard civics-textbook vocabulary. 'Checks and balances' is the more common phrase in everyday US speech. Invoked constantly during conflicts between courts and presidents, especially Trump 2017–2021 and 2025–. 'Constitutional crisis' (above) usually means the separation of powers is being tested.
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