A mortgage is a legal instrument of the common law which is used to create a security interest in real property held by a lender as a security for a debt, usually a mortgage loan. Hypothec is the corresponding term in civil law jurisdictions, albeit with a wider sense, as it also covers non-possessory lien.

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.

The word is a Law French term meaning "dead pledge," originally only referring to the Welsh mortgage (see below), but in the later Middle Ages was applied to all gages and reinterpreted by folk etymology to mean 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.