A stock market crash is a sudden dramatic loss of value of shares of stock in corporations. Crashes often follow speculative stock market bubbles such as the dot-com boom.
The most famous crash in 1929, (known as Black Thursday) when the Dow dropped 50%, preceded the Great Depression. The succeeding years saw the Dow drop a total of over 85%.
There was also a crash or "adjustment" on Monday October 19, 1987, known in financial circles as Black Monday, when the Dow Jones Industrial Average lost 22% of its value in one day, bringing to an end a five-year bull run. The FTSE lost 10.8% on that Monday and a further 12.2% the following day. The pattern was repeated across the world.