"Flash crash" is a term that refers to a sudden, very rapid price drop in the financial markets. This phenomenon can lead to a sharp drop in stock market indices in a very short period of time, often within minutes. A well-known example is the event of May 6, 2010, when the Dow Jones index on Wall Street fell by 9% in a few hours, resulting in the loss of 999 points, the largest absolute drop ever at that time.

Causes of a flash crash can vary, but they are often attributed to automatic trading algorithms, 'high frequency trading', erroneous transactions (such as a 'fat finger' input error), or large at-best sell orders. These computer-driven transactions can quickly trigger a chain reaction of price declines, causing the market to fluctuate sharply within moments.