A budget deficit occurs when an entity (often a government) spends more money than it takes in. The opposite of a budget deficit is a budget surplus. Debt is essentially an accumulated flow of deficits. In other words, a deficit is a flow and debt is a stock.

An accumulated deficit over several years (or centuries) is referred to as the government debt. Government debt is usually financed by borrowing, although if a government's debt is denominated in its own currency it can print new currency to pay debts. Monetizing debts, however, can cause rapid inflation if done on a large scale. Governments can also sell assets to pay off debt. Most governments finance their debts by issuing long-term government bonds or shorter term notes and bills. Many governments use auctions to sell government bonds.

Governments usually must pay interest on what they have borrowed. Governments reduce debt when their revenues exceed their current expenditures and interest costs. Otherwise, government debt increases, requiring the issue of new government bonds or other means of financing debt, such as asset sales.

According to Keynesian economic theories, running a fiscal deficit and increasing government debt can stimulate economic activity when a country's output (GDP) is below its potential output. When an economy is running near or at its potential level of output, fiscal deficits can cause inflation.

Primary deficit, total deficit, and debt

Main: Primary deficit

The government's deficit can be measured with or without including the interest it pays on its debt. The primary deficit is defined as the difference between current government spending and total current revenue from all types of taxes. The total deficit (which is often just called the 'deficit') is spending, plus interest payments on the debt, minus tax revenues.

Therefore, if G_t is government spending and T_t is tax revenue, then

If D_{t-1} is last year's debt, and r is the interest rate, then

Finally, this year's debt can be calculated from last year's debt and this year's total deficit:

Economic trends can influence the growth or shrinkage of fiscal deficits in several ways. Increased levels of economic activity generally lead to higher tax revenues, while government expenditures often increase during economic downturns because of higher outlays for social insurance programs such as unemployment benefits. Changes in tax rates, tax enforcement policies, levels of social benefits, and other government policy decisions can also have major effects on public debt. For some countries, such as Norway, Russia, and members of the Organization of Petroleum Exporting Countries (OPEC), oil and gas receipts play a major role in public finances.