U.S. Cabinet

The Cabinet of the United States is composed of the most senior appointed officers of the executive branch of the government serving under the President. Among those are the Vice President and the heads of the federal executive departments, all of whom are by federal law (3 U.S.C. § 19) in the line of succession to the presidency and have duties under the 25th Amendment to the Constitution. Aside from the Attorney General (and the Postmaster General back when it headed an executive department), the heads of the executive departments all receive the title of Secretary.

In addition, the President can by custom unilaterally designate senior White House staffers, heads of other federal agencies and the Ambassador to the United Nations as members of the Cabinet, although this is a symbolic status marker and does not, apart from attending cabinet meetings, confer any additional powers such as mentioned above.

All members of the Cabinet (except for the Vice President, who is elected under the same procedures as the President) serve at the pleasure of the President, who can dismiss them at will for no cause.

...
Wikipedia