Black Monday
The most notorious day in financial history (Oct 19, 1987). The DJIA fell 508 points, almost 22%.
Investopedia Commentary
Since Black Monday, there have been multiple mechanisms built into the market to prevent panic selling such as trading curbs and circuit breakers.
Related Links
Cyclical Versus Non-Cyclical Stocks
The Greatest Market Crashes
See also: Circuit Breaker, Correction, Dow Jones Industrial Average - DJIA, Kondratiev Wave, New York Stock Exchange - NYSE, Trading Curb
Black Monday