What is a second mortgage?

A second mortgage is simply an additional mortgage registered against the title of your home. Some lenders call it a "Home Equity Loan" or "Home Equity Line of Credit" and since these types of loans are registered against the title of your home as a second charge - they are all second mortgages. First and second mortgages are characterized by when they were registered on title respectfully, not by how much money was advanced or is owing.