A second mortgage typically refers to a secured loan (or mortgage) that is subordinate to another loan against the same property.

In real estate, a property can have multiple loans or liens against it. The loan which is registered with county or city registry first is called the first mortgage or first position trust deed. The lien registered second is called the second mortgage. A property can have a third or even fourth mortgage, but those are rarer.

Second mortgages are called subordinate because, if the loan goes into default, the first mortgage gets paid off first before the second mortgage gets any money. Thus, second mortgages are riskier for the lender, who generally charges a higher interest rate.

In most cases, a second mortgage takes the form of a home equity loan and the two are synonymous, from a financial standpoint. The difference in terminology is that a mortgage traditionally refers to the legal lien instrument, rather than the debt itself.