The term agency is defined in different ways in different contexts for the budgetary aspects of the law. In general, any instrumentality of the Federal Government is considered an “agency”. While the term applies to the Executive Branch, differences may occur as to its applicability to Congress or the Judicial Branch. In addition, private entities which serve some form of governmental purpose such as government sponsored enterprises (GSEs), while generally not considered “agencies” become more difficult to classify if they circumstances occur such as the Federal National Mortgage Administration (Fannie Mae) being considered “on-budget” by the Congressional Budget Office (though not by the Office of Management and Budget).
No one definition of this term has general, governmentwide applicability. “Agency” and related terms, like “executive agency” or “federal agency,” are defined in different ways in different laws and regulations. For example, the provisions of the Budget and Accounting Act of 1921 relating to the preparation of the President’s budget specifically define “agency” to include the District of Columbia government but exclude the legislative branch or the Supreme Court (31 U.S.C. § 1101).
CHAPTER 11-THE BUDGET AND FISCAL, BUDGET, AND PROGRAM INFORMATION
(a) In this section, “agency” means a department, agency, or instrumentality of the United States Government except a mixed-ownership Government corporation.
Note this similar definition in section 1108 of the the same chapter of title 31 of the U.S. Code, but without including the exception for “a mixed-ownership Government corporation”:
SEC. 1108. PREPARATION AND SUBMISSION OF APPROPRIATIONS REQUESTS TO THE PRESIDENT
(a) In this section (except subsections (b)(1) and (e)), “agency” means a department, agency, or instrumentality of the United States Government.
SUBCHAPTER II – ADMINISTRATIVE PROCEDURE
For the purpose of this subchapter-
(1) “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;