What is Federal Funds?

Legal Definition
In the United States, federal funds are overnight borrowings between banks and other entities to maintain their 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 one day only, that is, "overnight". The interest rate at which these deals are done is called the federal funds rate. Federal funds are not collateralized; like eurodollars, they are an unsecured interbank loan.

Federal funds transactions by regulated financial institutions neither increase nor decrease total bank reserves. Instead, they redistribute reserves. Before 2008, this meant that otherwise idle funds could yield a return. (Since 2008,the Fed has paid interest on reserves, including excess reserves.) Banks may borrow these funds to avoid an overdraft (that is, the balance going below reserve requirement) of their reserve account, or in order to meet the reserves required to back their deposits. Federal funds are definitive money, meaning that they are available for immediate spending, while checks and many other forms of money must be cleared by banks and typically take several days before becoming available for spending.

Participants in the federal funds market include commercial banks, savings and loan associations, government-sponsored enterprises, branches of foreign banks in the United States, federal agencies, and securities firms. Many relatively small institutions that accumulate reserves in excess of their requirements lend reserves overnight to money center and large regional banks, as well as to foreign banks operating in the United States. Federal agencies also lend idle funds in the federal funds market.
-- Wikipedia
Legal Definition
In the United States, unsecured LOANS available to member BANKS from excess balances held at one of the 12 FEDERAL RESERVE BANKS. The process of borrowing Federal Funds is represented as a purchase of funds, while lending is the sale of funds. The majority of Federal Funds transactions mature the business day after they are contacted, though term transactions for from one week to six months can be arranged. Federal Funds are available immediately (i.e., they are considered GOOD MONEY), an advantage over interbank CLEARINGHOUSE funds, which are generally only accessible one to two days after contracting.