Definition: A fully indexed option payment is a principal and interest payment. The interest rate is computed by adding the monthly index rate to the mortgage loan's fixed margin rate. Fully indexed payments are a feature of an Option ARM mortgage loan. If the borrower pays the fully indexed loan payment every month on a 30-year loan, two things will happen. First, the loan balance will reduce every month. Second, the loan will be paid off at the end of 30 years.

