<human>:  What is the definition of a "black swan event"?
<bot>: A black swan event is a term coined by statistician Nassim Nicholas Taleb to describe an event that is extraordinarily rare, has immense impact, and is typically considered after the fact to have been predictable. A black swan event can be either positive or negative and usually carries long-term consequences. Examples of black swan events include wars, natural disasters, technological breakthroughs, and major world events.