Debt is an obligation that requires one party, the debtor, to pay money borrowed or otherwise withheld from another party, the creditor. Debt may be owed by a sovereign state or country, local government, company, or an individual. Commercial debt is generally subject to contractual terms regarding the amount and timing of repayments of principal and interest. Loans, bonds, notes, and mortgages are all types of debt. In financial accounting, debt is a type of financial transaction, as distinct from equity.

The term can also be used metaphorically to cover moral obligations and other interactions not based on a monetary value. For example, in Western cultures, a person who has been helped by a second person is sometimes said to owe a "debt of gratitude" to the second person.

The English term "debt" was first used in the late 13th century and comes by way of Old French from the Latin verb debere, "to owe; to have from someone else." The related term "debtor" was first used in English also in the early 13th century.

Principal is the amount of money originally invested or loaned, on which basis interest and returns are calculated.

There are three main ways repayment may be structured: the entire principal balance may be due at the maturity of the loan; the entire principal balance may be amortized over the term of the loan; or the loan may be partially amortized during its term, with the remaining principal due as a "balloon payment" at maturity. Amortization structures are common in mortgages and credit cards.

Debtors of every type default on their debt from time to time, with various consequences depending on the terms of the debt and the law governing default in the relevant jurisdiction. If the debt was secured by specific collateral, such as a car or house, the creditor may seek to repossess the collateral. In more serious circumstances, individuals and companies may go into bankruptcy.

Common types of debt owed by individuals and households include mortgage loans, car loans, credit card debt, and income taxes. For individuals, debt is a means of using anticipated income and future purchasing power in the present before it has actually been earned. Commonly, people in industrialized nations use consumer debt to purchase houses, cars and other things too expensive to buy with cash on hand.

People are likely to spend more and get into debt when they use credit cards as against cash to buy products and services. This is primarily because of the transparency effect and consumer's "pain of paying." The transparency effect refers to the idea that the further you are from cash (as with a credit card or other forms of payment), the less transparent it is and the less aware you are of how much you have spent. The less transparent or further away from cash the form of payment employed is, the less an individual feels the "pain of paying" and thus is likely to spend more. Furthermore, the differing physical appearance/form that credit cards have from cash may cause them to be viewed as "monopoly" money vs. real money, luring individuals to spend more money than they would if they only had cash available.