Black Monday: The day the Dow Jones Industrial Average (DJIA) lost almost 22 percent, marking the start of a global stock market decline. Occurring on October 19, 1987, Black Monday was one of the most dramatic days in financial history. By the end of that month, most of the major exchanges had declined by 20 percent.

The cause of the Black Monday has not been attributed to any single news event. While there are many theories offering various explanations, most agree that mass panic caused the crash to escalate. Since 1987, a number of protective mechanisms have been established to prevent panic selling.

Source: Investopedia