What is Discount Window?

Legal Definition
The discount window is an instrument of monetary policy (usually controlled by central banks) that allows eligible institutions to borrow money from the central bank, usually on a short-term basis, to meet temporary shortages of liquidity caused by internal or external disruptions. The term originated with the practice of sending a bank representative to a reserve bank teller window when a bank needed to borrow money.

The interest rate charged on such loans by a central bank is called the discount rate, base rate, or repo rate, and is separate and distinct from the prime rate. It is also not the same thing as the federal funds rate or its equivalents in other currencies, which determine the rate at which banks lend money to each other. In recent years, the discount rate has been approximately a percentage point above the federal funds rate (see Lombard credit). Because of this, it is a relatively unimportant factor in the control of the money supply and is only taken advantage of at large volume during emergencies.
-- Wikipedia
Legal Definition
In the United States, a facility made available by the FEDERAL RESERVE BANKS that permits authorized BANKS to borrow shortterm funds on an emergency basis in order to balance cash outflows.