US stock market adds $1.2T in single-day surge

US stock market: The US stock market refers to the combined value of publicly listed companies trading on major US exchanges such as the New York Stock Exchange and Nasdaq, typically tracked through benchmark indices like the S&P 500, Dow Jones Industrial Average, and Nasdaq Composite. In the news item, the US stock market is described as having a large single‑day increase in aggregate market capitalization, reflecting a broad rally in major indices led by technology and AI‑related stocks. Market_rally: Major US stock indices including the S&P 500, Nasdaq Composite, and Dow Jones Industrial Average rose meaningfully on September 21, 2026, with tech shares among the strongest performers. AI_and_chip_stocks: AI‑related semiconductor companies such as Advanced Micro Devices and Nvidia led gains, with AMD reaching a landmark valuation milestone as investors focused on its role in artificial intelligence computing. Valuation_backdrop: Recent analysis notes that valuation measures such as the cyclically adjusted price‑to‑earnings ratio are near historically elevated levels, indicating the US stock market is highly valued relative to corporate profits.