The national debt of the United States is the total national debt owed by the federal government of the United States to Treasury security holders. The national debt at any point in time is the face value of the then-outstanding Treasury securities that have been issued by the Treasury and other federal agencies. The terms "national deficit" and "national surplus" usually refer to the federal government budget balance from year to year, not the cumulative amount of debt. In a deficit year the national debt increases as the government needs to borrow funds to finance the deficit, while in a surplus year the debt decreases as more money is received than spent, enabling the government to reduce the debt by buying back some Treasury securities. In general, government debt increases as a result of government spending and decreases from tax or other receipts, both of which fluctuate during the course of a fiscal year. There are two components of gross national debt:"Debt held by the public" – such as Treasury securities held by investors outside the federal government, including those held by individuals, corporations, the Federal Reserve, and foreign, state and local governments. "Debt held by government accounts" or "intragovernmental debt" – is non-marketable Treasury securities held in accounts of programs administered by the federal government, such as the Social Security Trust Fund. Debt held by government accounts represents the cumulative surpluses, including interest earnings, of various government programs that have been invested in Treasury securities.

A country's gross government debt is the financial liabilities of the government sector. Changes in government debt over time reflect primarily borrowing due to past government deficits. A deficit occurs when a government's expenditures exceed revenues. Government debt may be owed to domestic residents, as well as to foreign residents. If owed to foreign residents, that quantity is included in the country's external debt.