tail event

noun3

1. •
   a low-probability event

   • 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
2. •
   an 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)
3. •
   an event that initiates an activity