Black Swan events are rare, impactful, and retrospectively predictable
A Black Swan is an event or occurrence that is highly improbable, has a major impact, and is retrospectively predictable. The term comes from the metaphor of a black swan, which was once thought to be an impossible occurrence, as all swans were believed to be white. The term is used to describe events such as stock market crashes, natural disasters, or other unforeseen events that have a significant impact on individuals, organizations, or societies. Black Swans are characterized by their rarity, unpredictability, and the fact that they are often not anticipated by conventional thinking or models.
Before the discovery of Australia, people in the Old World were convinced that all swans were white, an unassailable belief as it seemed completely confirmed by empirical evidence. The sighting of the first black swan might have been an interesting surprise for a few ornithologists (and others extremely concerned with the coloring of birds), but that is not where the significance of the story lies. It illustrates a severe limitation to our learning from observations or experience and the fragility of our knowledge. One single observation can invalidate a general statement derived from millennia of confirmatory sightings of millions of white swans. All you need is one single (and, I am told, quite ugly) black bird.
…
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 occurrence after the fact, making it explainable and predictable.
source: The Black Swan – Taleb (2010), § “Prologue”
see also: