Executive Departments

The United States federal executive departments are among the oldest primary units of the executive branch of the federal government of the United States—the Departments of State, War, and the Treasury all being established within a few weeks of each other in 1789.

The heads of the federal executive departments are the members of the traditional Cabinet; since 1792, they have, by statutory specification, constituted a line of succession, after the Speaker of the House and the president pro tempore of the Senate to the presidency in the event of a vacancy in both that office and the vice presidency. The Constitution refers to these officials when it authorizes the President, in Article II, section 2, to "require the Opinion, in writing, of the principal Officer in each of the executive Departments, upon any Subject relating to the Duties of their respective Offices." In brief, they and their organizations are the administrative arms of the president.

ref: Wikipedia - United States federal executive departments