What was the nickname for the severe U.S. stock-market decline that began in October 1929?

The severe U.S. stock-market decline that began in October 1929 was called the Wall Street Crash of 1929.

The crash followed years of strong share-price growth, widespread speculation, and buying stocks with borrowed money. In September 1929, prices began weakening, and confidence deteriorated rapidly. The most famous trading sessions were Black Thursday on October 24, Black Monday on October 28, and Black Tuesday on October 29.

The collapse did not by itself cause every part of the Great Depression, but it sharply damaged confidence, wealth, and investment. Banks, businesses, and households faced falling asset values and tightening credit. The economic downturn then spread internationally through trade and financial links.

The 1929 crash is often confused with the Great Depression itself. The crash was a market event; the Great Depression was the much longer worldwide economic crisis that followed.