black swan

Short definition from Wiktionary

Noun

1. 1
   Cygnus atratus, a swan with black plumage and a red bill which is endemic to Australia.
2. 2
   Something believed impossible or not to exist, especially if an example is subsequently found; also, something extremely rare; a rara avis.
3. 3
   A rare and hard-to-predict event with major consequences.