What does the phrase separation of powers mean in reference?

The "separation of powers" employed by the US government refers to the manner in which authority is divided between three branches: the executive, the legislative and the judicial. This distribution of power was mandated by the US Constitution and was intended to ensure that no one branch of government could become too powerful and threaten American freedoms. To accomplish this, a unique system of checks and balances was established that affords each branch of government authority over the others in specific areas. This creates an interdependence that, in theory, prevents any one branch from establishing dominance. Examples of this include the requirement that judicial nominees from the executive branch win confirmation from the legislative branch. Likewise, bills passed by the legislative branch require the President's signature to become law and, once signed, can be subject to judicial challenges. Perhaps the best example involves the limits placed on declaring war. While a President (leader of the executive branch) is the top commander of the US Military, the Constitution does not empower the President to declare war. That power was reserved for the US Congress (legislative branch) and thus this "separation of powers" requires both the executive and the legislative branches to reach agreement before military action is authorized.