Interest

The amount of money charged by a lender / paid by a borrower for the use of money over a given period of time is referred to as interest. Thus a man who borrows $100 for one year and repays the $100 plus an extra $10 to the lender, has paid $10 interest. Interest is often expressed as a percentage of the principal. Here the rate of interest would be 10% per year since $10/$100 = 10%.