Definition of 'Black Monday'
October 19, 1987, when the Dow Jones Industrial Average (DJIA) lost almost 22% in a single day. That event marked the beginning of a global stock market decline, making Black Monday one of the most notorious days in recent financial history. By the end of the month, most of the major exchanges had dropped more than 20%.
Investopedia explains 'Black Monday'
Interestingly enough, the cause of the massive drop cannot be attributed to any single news event because no major news event was released on the weekend preceding the crash. While there are many theories that attempt to explain why the crash happened, most agree that mass panic caused the crash to escalate.
Since Black Monday, a number of protective mechanisms have been built into the market to prevent panic selling, such as trading curbs and circuit breakers.