There's not much monetary policy can do to save the global economy — especially when some major central banks have already cut interest rates into the negative territory, economists say. An interest rate is the amount of interest due per period, as a proportion of the amount lent, deposited or borrowed (called the principal sum). The total interest on an amount lent or borrowed depends on the principal sum, the interest rate, the compounding frequency, and the length of time over which it is lent, deposited or borrowed. It is defined as the proportion of an amount loaned which a lender charges as interest to the borrower, normally expressed as an annual percentage. It is the rat The interest rate is the amount charged, expressed as a percentage of the principal, by a lender to a borrower for the use of assets. Monetary policy: Actions of a central bank or other agencies that determine the size and rate of growth of the money supply, which will affect interest rates.