What is Bank Secrecy Act?

Legal Definition
The Bank Secrecy Act of 1970 (or BSA, or otherwise known as the Currency and Foreign Transactions Reporting Act) requires financial institutions in the United States to assist U.S. government agencies to detect and prevent money laundering. Specifically, the act requires financial institutions to keep records of cash purchases of negotiable instruments, and file reports of cash purchases of these negotiable instruments of more than $10,000 (daily aggregate amount), and to report suspicious activity that might signify money laundering, tax evasion, or other criminal activities.
-- Wikipedia