A mortgage in which payments are fixed for a period of one to two years although interest rates may fluctuate from month to month depending on market conditions. If interest rates go down, more of the payment goes towards reducing the principal; if rates go up, a larger portion of the monthly payment goes towards covering the interest. Most open variable rate mortgages allow prepayment of any amount (with certain minimums) on any payment date, up to a maximum total amount per year.