A black swan is an event or occurrence that is deemed unpredictable, and therefore impossible to predict. The term was popularized by Nassim Taleb in his book of the same name, which discusses the unpredictability of rare events in finance and other fields.

Black swans can be positive or negative events; for example, a company's successful IPO may be seen as a positive black swan, while its bankruptcy would be seen as a negative black swan. Black swans are typically outside of the realm of normal expectations and thus difficult to predict.

The existence of black swans is often used to argue against the use of historical data in financial analysis. Many believe that past performance cannot reliably indicate future results, because it's impossible to know which unexpected events will occur and have a significant impact on future outcomes. This means that attempting to make decisions based on past trends may lead investors astray, as unforeseen events can cause these trends to reverse course abruptly.