What is Reserve Requirement?

Alternative Forms: Cash reserve ratio
Legal Definition
The reserve requirement (or cash reserve ratio) is a central bank regulation employed by most, but not all, of the world's central banks, that sets the minimum amount of reserves that must be held by a commercial bank.
-- Wikipedia
Legal Definition
The reserve requirement (or cash reserve ratio) is a central bank regulation employed by most, but not all, of the world's central banks, that sets the minimum amount of reserves that must be held by a commercial bank. The amount of required minimum reserves is generally determined by the central bank as being equal to no less than a specified percentage or fraction of the amount of deposit liabilities that the commercial bank owes to its customers. The commercial bank's reserves normally consist of cash owned by the bank and stored physically in the bank vault (vault cash), plus the amount of the commercial bank's balance in that bank's account with the central bank.

The required reserve ratio is sometimes used as a tool in monetary policy, influencing the country's borrowing and interest rates by changing the amount of funds available for banks to make loans with. Western central banks rarely increase the reserve requirements because it would cause immediate liquidity problems for banks with low excess reserves; they generally prefer to use open market operations (buying and selling government-issued bonds) to implement their monetary policy. The People's Bank of China uses changes in reserve requirements as an inflation-fighting tool, and raised the reserve requirement ten times in 2007 and eleven times since the beginning of 2010.

An institution that holds reserves in excess of the required amount is said to hold excess reserves.
-- Wikipedia