Black Monday was the nickname given to the sudden global stock-market plunge on October 19, 1987.
The phrase refers most famously to the Dow Jones Industrial Average's 22.6% one-day fall, but the shock was international. Major markets in Hong Kong, Australia, the United Kingdom, and Canada also suffered steep losses. The synchronized decline showed how increasingly connected financial markets had become.
Several pressures preceded the crash, including high valuations, rising interest rates, oil-price concerns, and worries about the US trade deficit. Trading technology also mattered. Portfolio insurance and program trading automatically generated sell orders as prices fell, magnifying the downward spiral.
Black Monday is not the same event as Black Tuesday in 1929. The 1929 label belongs to the Wall Street crash that helped mark the beginning of the Great Depression, whereas the 1987 crash was followed by policy intervention and a relatively faster market recovery. The name is therefore shared by different historical episodes.