United States Code
Title 5 - Government Organization and Employees
Chapter 5 - Administrative Procedure
Section 551 - Definitions
Currently, "agency" means each authority of the Government of the United States, whether or not it is within or subject to review by another agency, but does not include:
(A) the Congress;
(B) the courts of the United States;
(C) the governments of the territories or possessions of the United States;
(D) the government of the District of Columbia;
or except as to the requirements of section 552 of this title
(E) agencies composed of representatives of the parties or of representatives of organizations of the parties to the disputes determined by them;
(F) [courts martial' and military commissions;
(G) military authority exercised in the field in time of war or in occupied territory; or
(H) functions conferred by sections 1738, 1739, 1743, and 1744 of title 12; subchapter II of chapter 471 of title 49; or sections 1884, 1891–1902, and former section 1641(b)(2), of title 50, appendix; 1
1Sections 1884 and 1891–1902 of Title 50, appendix, referred to in par. (1)(H), were a part of the various Housing and Rent Acts which were classified to section 1881 et seq. of the former Appendix to Title 50, War and National Defense, and had been repealed or omitted from the Code as executed prior to the elimination of the Appendix to Title 50. See Elimination of Title 50, Appendix note preceding section 1 of Title 50. Section 1641 of title 50, appendix, referred to in par. (1)(H), was repealed by Pub. L. 87–256, §111(a)(1), September 21, 1961, 75 Stat. 538.
Source:
https://uscode.house.gov/view.xhtml?req=granuleid:USC-prelim-title5-section551&num=0&edition=prelim