text: variable interest rate: A variable interest rate is based on fluctuating rates in the banking system, such as the prime rate. For example, if on January 1, the prime rate was 6 percent and your credit card's variable rate formula was the prime rate plus 9.9 percent, your interest rate would be 15.9 percent (see prime rate).\n\nprime rate: The interest rate banks charge for loans to their biggest and highest-rated customers. The prime rate changes based on the demand for money and the rate the U.S. Federal Reserve Bank charges to its member banks. It is used as a major economic indicator.