Federal funds

n. (USA) reserve money lent and deposited overnight by one Federal Reserve Bank to another

This article is about funds maintained by the U.S. Federal Reserve. For the funds provided by the U.S. government in terms of aid and assistance, see Federal aid. In the United States, federal funds are bank reserves at the Federal Reserve. Banks keep reserves at Federal Reserve Banks to meet their reserve requirements and to clear financial transactions. Transactions in the federal funds market enable depository institutions with reserve balances in excess of reserve requirements to lend reserves to institutions with reserve deficiencies. These loans are usually made for 1 day only, i.e. "overnight." The interest rate at which these deals are done is called the federal funds rate.
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Non-interest bearing deposits held in reserve for depository institutions at their district Federal Reserve Bank. Also, excess reserves lent by banks to each other.