The discount rate at which a central bank repurchases government securities from the commercial banks, depending on the level of money supply it decides to maintain in the country's monetary system. To temporarily expand the money supply, the central bank decreases repo rates (so that banks can swap their holdings of government securities for cash). To contract the money supply it increases the repo rates. Alternatively, the central bank decides on a desired level of money supply and lets the market determine the appropriate repo rate. Repo is short for repossession.
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