The correct answer is: "division of powers".
The principle of division of powers, developed by Baron de Montesquieu during the Enlightenment Era, states that the power in a nation needs to be divided in three independent branches: legislative, executive and judiciary. For example, in the US Federal goverment these are, respectively, the US Congress, the US Goverment and the US Supreme Court.
Moreover, a system of Checks and Balances needs to implemented, so that each of the branches supervises the other two, so that none of them gathers enough power as to overrule the other two. It is the way of avoiding a tyrannical state or authoritarian practices.