Separation of powers is a constitutional principle that refers to the division of government powers and functions among separate and independent branches of government. The three main branches—the legislative, executive, and judicial—are assigned distinct responsibilities to ensure a system of checks and balances. This separation aims to prevent the abuse or consolidation of power by any one branch and safeguard individual rights and democratic governance. 

Montesquieu was a French philosopher and political theorist who proposed the theory of separation of powers in his influential work, "The Spirit of the Laws," published in 1748. His theory emphasizes the need to distribute governmental powers among different branches to prevent tyranny and preserve liberty. Montesquieu argued that the separation of powers into legislative, executive, and judicial branches would ensure a system of checks and balances. This would prevent the concentration of power in any single authority, promote accountability, and protect individual rights. Montesquieu's theory greatly influenced the development of modern democratic systems around the world.