Your own source answers your question quite well:
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.
The free dictionary defines it as "a phenomenon that occurs even though it had been thought to be impossible." Note it's lack of field limitation.
According to JSTOR, the phrase derives from a Latin expression; its oldest known occurrence is the poet Juvenal's characterization of something being "rara avis in terris nigroque simillima cygno" ("a rare bird in the lands and very much like a black swan").
Charles Dickens used the term in A Christmas Carol (19 December 1843):
Such a bustle ensued that you might have thought a goose the rarest of all birds; a feathered phenomenon, to which a black swan was a matter of course: and, in truth, it was something very like it in the house.
September 11, 2001 is a classic example of a Black Swan. It was only a failure of imagination by most Americans (including myself) to never have contemplated beforehand the possibility of such a dreadful day. - Christian Science Monitor
The above source gives a good history of the term black swan.
A typical reaction to a black swan (here is Ebenezer Scrooge himself):