It stands for systemically important bank; a bank (or a banking institution) that is identified by a regulatory authority as potentially important within the financial sector so that its collapse or failure would create a significant threat to the sector and the economy at large. This threat may take the form of a trigger of a wide-spread financial crisis (e.g., a chain effect of collapses of other institutions).

The systemic importance of a bank depends on a set of factors including its size, interlinkages with the other institutions in the financial system (systemic interconnectedness) and the broader economy and the substitutability of its products/ services, etc, as determined by the relevant authorities due to its potential role, if failed to orderly function or operate, to cause a significant disruption to the financial system and economic activity.

Systemically important banks (SIBs) are also colloquially known as too big to fail (TBTF).