A mortgage is the pledging of a property to a lender as a security for a mortgage loan. While a mortgage in itself is not a debt, it is evidence of a debt.

It is a transfer of an interest in land, from the owner to the mortgage lender, on the condition that this interest will be returned to the owner of the real estate when the terms of the mortgage have been satisfied or performed. In other words, the mortgage is a security for the loan that the lender makes to the borrower.

The term comes from the Old French "dead pledge," apparently meaning that the pledge ends (dies) either when the obligation is fulfilled or the property is taken through foreclosure.

In most jurisdictions mortgages are strongly associated with loans secured on real estate rather than other property (such as ships) and in some jurisdictions only land may be mortgaged. Arranging a mortgage is seen as the standard method by which individuals and businesses can purchase residential and commercial real estate without the need to pay the full value immediately. See mortgage loan for residential mortgage lending, and commercial mortgage for lending against commercial property.

The measurement of a mortgage with regards to cost to the borrower can be measured by Annual Percentage Rate (APR) or many other formulas for true cost such as Lender Police Effective Annual Rate.

A home buyer or builder can obtain financing (a loan) either to purchase or secure against the property from a financial institution, such as a bank, either directly or indirectly through intermediaries.

Features of mortgage loans such as the size of the loan, maturity of the loan, interest rate, method of paying off the loan, and other characteristics can vary considerably.

A commercial mortgage is similar to a residential mortgage, except the collateral is a commercial building or other business real estate, not residential property.

In addition, commercial mortgages are typically taken on by businesses instead of individual borrowers. The borrower may be a partnership, incorporated business, or limited company, so assessment of the creditworthiness of the business can be more complicated than is the case with residential mortgages.

Some commercial mortgages are nonrecourse, that is, that in the event of default in repayment, the creditor can only seize the collateral, but has no further claim against the borrower for any remaining deficiency.

The general reason for this is twofold: many laws significantly prevent the creditor from going after the borrower for any deficiency, and mortgages structured for sale as bonds give a higher priority to constantly receiving some sort of income and therefore require a clause which allows the lender to take the property immediately, regardless of bankruptcy proceedings that the borrower might be going through.

Frequently, the mortgage is supplemented by a general obligation of the borrower or a personal guarantee from the owner(s), which makes the debt payable in full even if foreclosure on the mortgaged collateral does not satisfy the outstanding balance.