What is Federal Funds Rate?

Legal Definition
In the United States, the federal funds rate is the interest rate at which depository institutions (banks and credit unions) lend reserve balances to other depository institutions overnight, on an uncollateralized basis. Reserve balances are amounts held at the Federal Reserve to maintain depository institutions' reserve requirements. Institutions with surplus balances in their accounts lend those balances to institutions in need of larger balances. The federal funds rate is an important benchmark in financial markets.

The interest rate that the borrowing bank pays to the lending bank to borrow the funds is negotiated between the two banks, and the weighted average of this rate across all such transactions is the federal funds effective rate.

The federal funds target rate is determined by a meeting of the members of the Federal Open Market Committee which normally occurs eight times a year about seven weeks apart. The committee may also hold additional meetings and implement target rate changes outside of its normal schedule.

The Federal Reserve uses open market operations to influence the supply of money in the U.S. economy to make the federal funds effective rate follow the federal funds target rate.
-- Wikipedia
Legal Definition
The rate funds are loaned between the federal reserve overnight. It is called fed funds rate.