Since the term government corporation is not specifically defined in law, it is not clearly delineated. In terms of budget law, the term is meaningful for a variety of reasons, not least because of the treatment of the receipts that such an entity may bring to the Federal Government. The distinction between governmental receipts (commonly termed “revenue”) and offsetting receipts is an important example. Revenue is produced through the sovereign power of the government, which means it has the power to compel individuals and entities to provide it with resources – usually money. Offsetting receipts (and other forms of collections) are defined as the government generating income through business-like activities, and hence the intersection with the concept of a government corporation. For a list, developed by the Congressional Research Service, of government corporations under its definition, see the Appendix from the CRS Report referred to here.
Three forms of government-related entity are relevant to the definition of government corporation: A wholly-owned government corporation, a mixed-ownership corporation, and a government-sponsored enterprise. Whether, how, and to what extent each of these should be considered for budgetary purposes tends to depend on the specific nature of each entity, though certain general traits can be identified.
An entity being called a “government corporation” suggests that it operates using a business model that exchanges goods or services for receipts. This is not always the case, as there are certain agencies of the Federal Government that may use the term “corporation” and do not have any real business like activity producing income. An example of this is the Millennium Challenge Corporation (see footnote #5 below from the CRS Report on the subject of government corporations below).
The Congressional Research Service issued a particularly helpful report related to Government Corporations describing their history, definition, development and context. The following excerpt is specifically related to issue of definition:
The Government Corporation: Contested Definitions
What is a federal government corporation, and what are the essential characteristics of a government corporation?
As defined in this report, a federal government corporation is an agency of the federal government, established by Congress to perform a public purpose, which provides a market-oriented product or service and is intended to produce revenue that meets or approximates its expenditures. By this definition, there are 17 entities that are government corporations. 
The U.S. Code does not provide a single definition of the term “government corporation.” Title 5 of the U.S. Code defines a “government corporation” as “a corporation owned or controlled by the Government of the United States” (5 U.S.C. 103).  Meanwhile, the Government Corporation Control Act ((GCCA) 31 U.S.C. 9101-10) states that the term “government corporation” means “a mixed-ownership Government corporation and a wholly-owned government corporation.” It then lists 28 entities—some, like the Pennsylvania Avenue Development Corporation, now defunct—as being “government corporations” for the purposes of chapter 91 of Title 31.
In addition to the enumeration of corporations provided in the GCCA, there have been several other listings of corporations available, each different and based upon the definition employed by the compiler. Corporations cover the spectrum from such large, well-known corporations as the United States Postal Service and the Federal Deposit Insurance Corporation to such small, low- visibility corporate bodies as the Federal Financing Bank in the Treasury Department and Federal Prison Industries (UNICOR) in the Justice Department.
The number of federal corporations is in moderate flux. New corporations are established from time to time (e.g., the Valles Caldera Trust in 2000),  and existent ones are dissolved (e.g., the Rural Telephone Bank in 2008). 
Government corporations should not be confused with quasi governmental entities, such as government-sponsored enterprises (GSEs). A GSE (e.g., Fannie Mae) is a privately owned, federally chartered financial institution with nationwide scope and lending powers that benefits from an implicit federal guarantee to enhance its ability to borrow money.  GSEs are important institutions worthy of separate analysis, but they are not discussed, except in passing, in this report.  Unlike government corporations, with GSEs and other quasi governmental entities, the legal and political lines of accountability can be ambiguous.  In 1996, for instance, the Office of Personnel Management (OPM) created the United States Investigation Services Corporation as an employee stock-ownership plan (ESOP), an entry into the quasi government category that sparked debate regarding its status and authority. 
5. For a list of federal government corporations, as defined in this report, please consult the Appendix. This report’s definition of “government corporation” excludes a great many federal entities. It excludes private corporations created by federal statute (e.g., the American National Red Cross, the Securities Investor Protection Corporation, and Fannie Mae). It also excludes some corporations that Congress itself has called “government corporations.” For example, the 108th Congress established the Millennium Challenge Corporation (MCC) as a “government corporation” (P.L. 108- 199). Though clearly a federal entity, the MCC is not included on this report’s list because MCC does not provide market-oriented products or services—the MCC is a grant-awarding agency that is not designed to be financially self- sufficient.
6. This definition holds only for “the purpose of this title,” i.e., Title 5 of the U.S. Code.
8. With Congress’s assent, both the Rural Telephone Bank and the Pennsylvania Avenue Development Corporation and dissolved themselves under their own corporate authorities.
9. In 1996, the board of directors of the Federal National Mortgage Association changed its name to Fannie Mae, although the law still refers to the organization by its former name.
10. For a discussion of GSEs, see CRS Report RS21663, Government-Sponsored Enterprises (GSEs): An Institutional Overview, by Kevin R. Kosar; and Thomas H. Stanton, Government-Sponsored Enterprises: Mercantilist Companies in the Modern World (Washington: AEI Press, 2002).
11. Harold Seidman, “The Quasi World of the Federal Government,” The Brookings Review, vol. 2, summer 1988, pp. 23-27; CRS Report RL30533, The Quasi Government: Hybrid Organizations with Both Government and Private Sector Legal Characteristics, by Kevin R. Kosar; Jonathan G.S. Koppel, The Politics of Quasi Government: Hybrid Organizations and the Dynamics of Bureaucratic Control (Cambridge, UK: Cambridge University Press, 2003); and Ronald C. Moe, “The Emerging Federal Quasi Government: Issues of Management and Accountability,” Public Administration Review, vol. 61, May/June 2001, pp. 290-312.
The congressionally chartered National Veterans Business Development Corporation (NVBDC) is a case in point. The 2004 Omnibus Appropriations Act (P.L. 108-447, Div. K, Sec. 146) declared the NVBDC, thought by some to be a government corporation, to be “a private entity” and “not an agency, instrumentality, authority, entity, or establishment of the United States Government.” The Department of Justice did not agree with this characterization. See Office of the Legal Counsel, United States Department of Justice, Memorandum for Jennifer Newstead, General Counsel, Office of Management and Budget, March 19, 2004.
12. U.S. General Accounting Office, Privatization of OPM’s Investigations Service, GAO/GGD-96-97 (Washington: GAO, 1996); Ronald P. Sanders and James Thompson, “Live Long and Prosper: How One Former Federal Organization Is Adjusting to Life After Government,” Government Executive, vol. 29, April 1997, pp. 51-53; Stephen Barr, “OPM, in a First, Acts to Convert an Operation into Private Firm,” Washington Post, April 14, 1996, p. A4; and Dan Broidy, The Iron Triangle: Inside the Secret World of the Carlyle Group (New York: John Wiley, 2003).
Government Accountability Office (GAO) Glossary Entries
The GAO Glossary includes entries related to government corporations, one related to “mixed-ownership” and the other “wholly-owned” government corporations. These are included below. The Glossary itself does not have entry for the term government corporation.
Mixed-Ownership Government Corporation
An enterprise or business activity designated by the Government Corporation Control Act (31 U.S.C. § 9101) or some other statute as a mixed-ownership government corporation. The fiscal activities of some mixed-ownership government corporations appear in the budget. The Federal Deposit Insurance Corporation (FDIC) is an example of such a corporation. (For distinctions, see Government-Sponsored Enterprise; Off-Budget; Wholly-Owned Government Corporation.)
Wholly-Owned Government Corporation
An enterprise or business activity designated by the Government Corporation Control Act of 1945 (31 U.S.C. § 9101) or some other statute as a wholly-owned government corporation. Each such corporation is required to submit an annual business-type statement to the Office of Management and Budget (OMB). Wholly-owned government corporations are audited by Government Accountability Office (GAO) as required by the Government Corporation Control Act, as amended (31 U.S.C. § 9105), and other laws. The Pension Benefit Guaranty Corporation is an example of a wholly- owned government corporation. Budget concepts call for any corporation that is wholly owned by the government to be included on-budget. (For distinctions, see Government-Sponsored Enterprise; Mixed-Ownership Government Corporation; Off- Budget.)