What is a 'black swan' event?

A 'black swan' is a term popularised by Nassim Nicholas Taleb to describe an event that is a major surprise, has a significant impact, and is often rationalised in hindsight as if it were expected. The core characteristic is its profound unpredictability and rarity, lying outside the realm of normal expectations and historical modelling. While many black swans have negative consequences (e.g., the 2008 financial crisis), the concept also applies to positive, game-changing events (e.g., the rapid rise of the internet). Therefore, its defining feature is not its outcome but its randomness and unexpected nature.

A. Black swan events can be positive or negative; their defining feature is extreme impact and unpredictability, not necessarily a negative outcome.
C. This is the opposite of a black swan event, which is, by its very nature, unpredictable and cannot be meticulously planned for.
D. This is also the opposite; black swan events are defined by their extreme rarity and are not regularly occurring phenomena.

1. Taleb, N. N. (2007). The Black Swan: The Impact of the Highly Improbable. Random House. In the Prologue (pp. xxi-xxii), Taleb defines a black swan event by three attributes: "First, it is an outlier, as it lies outside the realm of regular expectations... Second, it carries an extreme impact. Third, in spite of its outlier status, human nature makes us concoct explanations for its occurrence after the fact, making it explainable and predictable."

2. Aven, T. (2013). "On the meaning of a black swan in a risk context". Safety Science, 57, pp. 44-51. Section 2, "The Black Swan concept as defined by Taleb," clarifies that the event is a surprise to the observer and lies outside the set of events that are considered likely to occur. This directly supports the 'random or unexpected' nature. DOI: https://doi.org/10.1016/j.ssci.2013.01.016

3. MIT OpenCourseWare. (2014). 15.020 Competition in Telecommunications, Lecture 12: Risk Management. Massachusetts Institute of Technology. The lecture notes discuss risk categories, defining "black swans" as unknown unknowns—events that are completely unexpected and have a massive impact, reinforcing their random and unpredictable character.