An event or occurrence that deviates beyond what is normally expected of a situation and that would be extremely difficult to predict. This term was popularized by Nassim Nicholas Taleb's book "The Black Swan: The Impact of the Highly Improbable." Mr Taleb is a finance professor and former Wall Street trader. 
He took his title from the shock that Europeans experienced when they discovered black swans in Australia. Until then, their data told them that all swans were white, so the discovery was unexpected.
A black swan in markets is an event that has not occurred in the past, thus rendering useless risk management models based on historic data. Such a risk model would assume that all swans were white.
Mr Taleb suggests his idea has been misunderstood. The problem, he told the CFA Institute in 2008, is not that black swans occur often. Rather, it is that they have truly catastrophic and unpredictable effects when they do happen, and so risk managers should concentrate on guarding against them.