The full form of FFI is Foreign Financial Institution.

The term foreign financial institution means any foreign entity that is engaged in the business of accepting deposits, making, granting, transferring, holding, or brokering loans or credits, or purchasing or selling foreign exchange, securities, commodity futures or options, or procuring purchasers and sellers thereof, as principal or agent. It includes but is not limited to depository institutions, banks, savings banks, money service businesses, trust companies, securities brokers and dealers, commodity futures and options brokers and dealers, forward contract and foreign exchange merchants, securities and commodities exchanges, clearing corporations, investment companies, employee benefit plans, dealers in precious metals, stones, or jewels, and holding companies, affiliates, or subsidiaries of any of the foregoing. The term does not include the international financial institutions identified in 22 U.S.C. 262r(c)(2), the International Fund for Agricultural Development, the North American Development Bank, or any other international financial institution so notified by the Office of Foreign Assets Control.

FFIs that enter into an agreement with the IRS to report on their account holders may be required to withhold 30% on certain payments to foreign payees if such payees do not comply with FATCA