The story
Etymology from Wiktionary, CC BY-SA 4.0.
Today it means: 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) Full definition →
Etymology from Wiktionary, CC BY-SA 4.0.