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.
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.
Stock market crashes are driven by panic as much as by underlying economic factors. So long as the prospect of further daily drops in the value of stocks persists, a bear market, equity investorss can be expected to sell.