Separation of Powers/Checks and Balances.
The term "separation of powers" refers to the three branches of government; the legislative, executive and judicial branches as set up by our founding fathers in the U.S. Constitution. The principle of separation of powers had already been given a fair trail in most state constitutions and had proved sound.
The legislative branch makes the laws and consists of the members of Congress. The founding fathers were in a debate on whether to base the number of representatives on population, which would give more power to the more populated states, or to give each state the same number of representatives, which would give equal power to all states regardless of their populations. The Great Compromise resulted in the creation of two houses, with representation based on population in one and with equal representation in the other. Thus the House of Representatives and the Senate were created.
The executive branch enforces the laws and consists of the President of the United States. During George Washington's presidency, people had already realized that one person could not carry out the duties of the President without advice and assistance, so the founding fathers provided support for the President in the form of department heads (cabinet members), but the responsibility of all executive actions still lie solely on the President.
The judicial branch interprets the laws and consists of nine (originally there were six, but that was later changed) Supreme Court justices. There are also lower Federal courts, but they were not created by the Constitution, but were later created by Congress using the powers granted by the "necessary and proper" clause.
The three branches work separately and have their own individual powers, but have to rely on the other branches in order to accomplish their goals. Each branch "checks" the other two branches in some way, which "balances" the powers given to each branch.