A mortgage is the transfer of an interest in property (or the equivalent in law - 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 the 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 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.

This 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 on other property (such as ships) and in some jurisdictions only land may be mortgaged. A mortgage is the standard method by which individuals and businesses can purchase real estate without the need to pay the full value immediately from their own resources. See mortgage loan for residential mortgage lending, and commercial mortgage for lending against commercial property.

The cost to the borrower is measured by the annual percentage rate (APR), which is an effective annual rate of interest and fees paid by the borrower.

In many countries, though not all (Iran) or (Bali, Indonesia is one exception), it is normal for home purchases to be funded by a mortgage. Few individuals have enough savings or liquid funds to enable them to purchase property outright. In countries where the demand for home ownership is highest, strong domestic markets have developed, notably in Ireland, Spain, the United Kingdom, Australia and the United States.