What is the separation of powers under the constitution?

Separation of powers is a political doctrine originating in the writings of Charles de Secondat, Baron de Montesquieu in The Spirit of the Laws, in which he argued for a constitutional government with three separate branches, each of which would have defined abilities to check the powers of the others. This philosophy heavily influenced the writing of the United States Constitution, according to which the Legislative, Executive, and Judicial branches of the United States government are kept distinct in order to prevent abuse of power. This United States form of separation of powers is associated with a system of checks and balances.

The principle of separation of powers provides checks and balances of power in the government, to prevent any one branch of government from taking over too much power.   Powers are separated between the executive, legislative, and judicial branches in government.

Context/detail:

The "Separation of Powers" principle 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.

Within his treatment of how governments will function best, Montesquieu argued that executive, legislative, and judicial functions of government ought to be divided between parts of the government, so that no one person or division of the government can infringe on the overall rights of others in the government or of the members of the society overall.

i would say that the politicial parties from state to state varies.