For the purpose of this subchapter—

(1) “agency” means—

(A) an Executive agency;

(B) the judicial branch;

(C) the Library of Congress;

(D) the Government Publishing Office; and

(E) the government of the District of Columbia;

(2) “employee” means an individual employed in or under an agency;

(3) “head of each agency” means—

(A) the Director of the Administrative Office of the United States Courts with respect to the judicial branch; and

(B) the Mayor of the District of Columbia with respect to the government of the District of Columbia; and

(4) “United States”, when used in a geographical sense, means the several States and the District of Columbia.