Which constitutional principle describes the division of power among the executive, legislative, and judicial ranches of government?

We're talking here about the "Separation of Powers" principle.  It was an idea embedded into the plans for American government by our founding fathers, based on their reading of Enlightenment political theory.  The terminology "separation of powers" was introduced by Charles-Louis de Secondat, the Baron of Montesquieu.  (Usually he's referred to as just "Montesquieu.")  He wrote an important work of political theory called "The Spirit of the Laws," published in 1748.