Nvidia's stock is down today primarily because China's internet regulator has banned the country's largest technology companies, like Alibaba and Bytedance, from purchasing Nvidia's AI chips. This ban includes a chip model Nvidia developed to comply with U.S. export rules for the Chinese market. This move has increased investor concerns about Nvidia's access to a key market and added to regulatory challenges in China, negatively impacting the stock price. Additionally, the broader tech sector and Nasdaq index have been affected by this news. The Federal Reserve's interest rate decision, which cut rates less than expected, also contributed to market caution, but the main reason for Nvidia's decline is the China AI chip ban and related market fears.

Nvidia fell about 2.6% today with shares trading around $170.29, reflecting these developments.