In finance, default is failure to meet the legal obligations (or conditions) of a loan, for example when a home buyer fails to make a mortgage payment, or when a corporation or government fails to pay a bond which has reached maturity. A national or sovereign default is the failure or refusal of a government to repay its national debt.

The biggest private default in history is Lehman Brothers, with over $600 billion when it filed for bankruptcy in 2008. The biggest sovereign default is Greece, with $138 billion in March 2012
The term "default" should be distinguished from the terms "insolvency", illiquidity and "bankruptcy":

Default: A debtor has passed the payment deadline on a debt they were due to pay.
Illiquidity: A debtor has insufficient cash (or other "liquefiable" assets) to pay debts.
Insolvency: A legal term meaning a debtor is unable to pay their debts.
Bankruptcy: A legal finding that imposes court supervision over the financial affairs of those who are insolvent or in default.
Default can be of two types: debt services default and technical default. Debt service default occurs when the borrower has not made a scheduled payment of interest or principal. Technical default occurs when an affirmative or a negative covenant is violated.

Affirmative covenants are clauses in debt contracts that require firms to maintain certain levels of capital or financial ratios. The most commonly violated restrictions in affirmative covenants are tangible net worth, working capital/short term liquidity, and debt service coverage.

Negative covenants are clauses in debt contracts that limit or prohibit corporate actions (e.g. sale of assets, payment of dividends) that could impair the position of creditors. Negative covenants may be continuous or incurrence-based. Violations of negative covenants are rare compared to violations of affirmative covenants.

With most debt (including corporate debt, mortgages and bank loans) a covenant is included in the debt contract which states that the total amount owed becomes immediately payable on the first instance of a default of payment. Generally, if the debtor defaults on any debt to the lender, a cross default covenant in the debt contract states that that particular debt is also in default.

In corporate finance, upon an uncured default, the holders of the debt will usually initiate proceedings (file a petition of involuntary bankruptcy) to foreclose on any collateral securing the debt. Even if the debt is not secured by collateral, debt holders may still sue for bankruptcy, to ensure that the corporation's assets are used to repay the debt.

There are several financial models for analyzing default risk, such as the Jarrow-Turnbull model, Edward Altman's Z-score model, or the structural model of default by Robert C. Merton (Merton Model).