Lehman Brothers was the New York investment bank that failed in September 2008, intensifying the global financial crisis.
Lehman filed for bankruptcy protection on September 15, 2008, after suffering enormous losses tied largely to the U.S. housing market and mortgage-related securities. Its collapse was the largest bankruptcy filing in U.S. history at that time and became a defining moment of the 2008 financial crisis.
The failure shocked markets because Lehman was deeply connected to banks, investors, insurers, and trading partners around the world. Its bankruptcy increased fears that other financial institutions could fail, causing credit markets to seize up. Governments and central banks then expanded emergency lending, guarantees, and rescue programs.
Bear Stearns had already collapsed earlier in 2008 and was acquired by JPMorgan Chase. Merrill Lynch was bought by Bank of America, while Washington Mutual failed later that month. These events are related, but Lehman Brothers is the canonical answer to this question.