Nassam Nicholas Taleb describes a Black Swan as having three properties:

First, it is an outlier, as it lies outside the realm of regular expectations, because nothing in the past can convincingly point to its possibility. Second, it carries an extreme impact. Third, in spite of its outlier status, human nature makes us concoct explanations for its occurence after the face, making it explainable and predictable.

Taleb neatly divides the world up into mediocristan and extremistan in terms of probability. For instance your chances of being a rock star, a millionare author, etc are very small, infintisimally so where luck plays more in your ascent than talent; this is his description of extremistan. Whereas mediocristan falls within knowable, even gaussian, probabilistic maths; such as human height, etc. Consequently events that inhabit 'extremistan' cannot be predicted even though their are whole industries and professions devoted to providing confidence in this area; such as economists, bankers, etc.

Taleb's thesis can be summed up that 'no-one is a good predictor of anything'. However he differentiates between negative and positive Black Swans. Lasers for instance were a positive Black Swan as they are used in all manner of technology other than what they were intended or originally designed for. However a negative Black Swan for a certain casino - outside of their normal assumptions of risk management - was one of their stars getting mauled by a white tiger. This apparently cost them 100 million, far more than cheating, or any other assumption of risk.