a mainly black swan with white flight feathers which is common in Australia and Tasmania and has been introduced elsewhere.
Cygnus atratus, family Anatidae
an unpredictable or unforeseen event, typically one with extreme consequences: [as modifier]:geopolitical black swan events, such as the Arab Spring and the Japanese earthquake, have further complicated the market dynamics
archaic something extremely rare:husbands without faults, if such black swans there be
[proverbial, from Juvenal's Satires]
black swan in other Oxford dictionaries
Definition of black swan in the US English dictionary