A mortgage is the transfer of an interest in property (or in law the equivalent - a charge) to a lender as a security for a debt - usually a loan of money. While a mortgage in itself is not a debt, it is lender's security for a debt. It is a transfer of an interest in land (or the equivalent), 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 (LPEAR).

In many countries it is normal for home purchases to be funded by a mortgage. In countries where the demand for home ownership is highest, strong domestic markets have developed, notably in Spain, the United Kingdom, Australia and the United States.