black swan

(名詞) 極度罕見且出乎意料但會造成重大影響的事件。
black-swan (形容詞):極度罕見且出乎意料但會造成重大影響的。Black swan = black-swan event。

Black swans are, of course, those highly improbable but painfully consequential events that strike from the blue — or from the streets of Cairo, or from an offshore oil rig, or from a poorly designed car part. They can destroy a company's reputation, cripple its financial performance, and perhaps even kill it outright. Because they are rare and almost impossible to predict, black-swan events tend to fall outside the scope of most companies' risk-management programs (assuming a company has such a program at all).
—Russ Banham, "Disaster Averted?," CFO Magazine, April 1, 2011

Like the risk models that failed then, the standard deviation in a Sharpe ratio would use historic data on volatility and assume a normal distribution for market moves. So-called black swan events (rare and unexpected), like America's first ever steep decline in house prices, can and will occur.
—Rob Cox and Antony Currie, "Treating Traders Like Hedge Funds," The New York Times, May 12, 2011