Open Market Operations

The term refers to routine activity by central banks in financial markets to influence the volume of money and credit in an economy, often by means of the sale or purchase of short term government securities in the domestic money market. Sales of government securities by a central bank, known as repurchase agreements or repos, drain cash from commercial banks, reducing their reserves and limiting credit expansion. The purchase by a central bank of such paper from commercial banks, known as a reverse repo, adds to banks' reserves and injects cash and credit into the economy.

See also: http://www.federalreserve.gov/monetarypolicy/bst_openmarketops.htm