Post-Crisis international financial regulation is animated by the buzzwords 'financial stability, " but surprisingly little attention has been paid to what these buzzwords actually mean. This Article argues that there are many-largely unexplored- disagreements regarding the meaning of 'financial stability, " and that this lack of consensus has the potential to cause a host of problems. Chief amongst these is that disagreement about the meaning of "financial stability" can thwart harmonized national implementation of international financial stability regulation. To draw attention to this largely-ignored definitional problem, and to start the process of addressing it, this Article proposes a working definition of 'financial stability." The proposed definition reflects technical notions about the state of financial institutions and markets during periods of stability, as well as a value-based assessment of the financial system as a means to broader global economic prosperity, rather than an end in itself.