Interest Rate:
A rate which is charged or paid for the use of money. An interest rate is often expressed as an annual percentage of the principal. It is calculated by dividing the amount of interest by the amount of principal. Interest rates often change as a result of inflation and Federal Reserve policies. For example, if a lender (such as a bank) charges a customer $90 in a year on a loan of $1000, then the interest rate would be 90/1000 *100% = 9%.