What is the separation of powers?

The separation of powers is the principle of dividing political power so it does not rest with one individual or faction.

The purpose is to centralize power only so far as necessary for the government to carry out its purposes — the protection of individual rights; and to decentralize it as much as possible keeping as much power as possible with the individual.

Such power can be divided vertically (federalism) and horizontally (dividing the government into separate branches).