tail event

plural tail events

1. mathematicsan event associated with a given infinite sequence Y=y_k|k>k_0 that is determined by any subsequence of the form Y_n=y_k|k>n,n>k_0 (a "tail" of that sequence)
2. a low-probability event

   ... When the MS index is high ex ante, subsequent large stock market losses and crashes are no longer tail events at all—rather they are perfectly expected events.

   2013, Mark Spitznagel, The Dao of Capital: Austrian Investing in a Distorted World, John Wiley & Sons, →ISBN, page 243

   2 more examples

   Exactly two years later, it was shattered by the force of a tsunami equivalent to the impact of 250 jumbo jets flying at 1,000 kilometers an hour—the very apotheosis of a real tail event.

   2012, Masahiro Kawai, Eswar S. Prasad, New Paradigms for Financial Regulation: Emerging Market Perspectives, Brookings Institution Press, →ISBN, page 186

   Policy frameworks anticipating only “ordinary” shocks within the usual economic cycles may not be very effective in mitigating the impact of such tail events when they actually materialize.

   2012, Masahiro Kawai, Eswar S. Prasad, New Paradigms for Financial Regulation: Emerging Market Perspectives, Brookings Institution Press, →ISBN, page 185

3. an event that initiates an activity