What is Bank Run?

Legal Definition
A bank run (also known as a run on the bank) occurs when in a fractional-reserve banking system (where banks normally only keep a small proportion of their assets as cash), a large number of customers withdraw cash from deposit accounts with a financial institution at the same time because they believe that the financial institution is, or might become, insolvent; and keep the cash or transfer it into other assets, such as government bonds, precious metals or stones. When they transfer funds to another institution it may be characterised as a capital flight. As a bank run progresses, it generates its own momentum: as more people withdraw cash, the likelihood of default increases, triggering further withdrawals. This can destabilize the bank to the point where it runs out of cash and thus faces sudden bankruptcy. To combat a bank run, a bank may limit how much cash each customer may withdraw, suspend withdrawals altogether, or promptly acquire more cash from other banks or from the central bank, besides other measures.

A banking panic or bank panic is a financial crisis that occurs when many banks suffer runs at the same time, as people suddenly try to convert their threatened deposits into cash or try to get out of their domestic banking system altogether. A systemic banking crisis is one where all or almost all of the banking capital in a country is wiped out. The resulting chain of bankruptcies can cause a long economic recession as domestic businesses and consumers are starved of capital as the domestic banking system shuts down. According to former U.S. Federal Reserve chairman Ben Bernanke, the Great Depression was caused by the Federal Reserve System, and much of the economic damage was caused directly by bank runs. The cost of cleaning up a systemic banking crisis can be huge, with fiscal costs averaging 13% of GDP and economic output losses averaging 20% of GDP for important crises from 1970 to 2007.

Several techniques have been used to try to prevent bank runs or mitigate their effects. They have included a higher reserve requirement (requiring banks to keep more of their reserves as cash), government bailouts of banks, supervision and regulation of commercial banks, the organization of central banks that act as a lender of last resort, the protection of deposit insurance systems such as the U.S. Federal Deposit Insurance Corporation, and after a run has started, a temporary suspension of withdrawals. These techniques do not always work: for example, even with deposit insurance, depositors may still be motivated by beliefs they may lack immediate access to deposits during a bank reorganization.
-- Wikipedia
Legal Definition
A sudden withdrawal of depositors who no longer trust their bank or economy. Today this is done with electronic transfers.