Exacerbated by high-frequency traders using strategies that have since been banned, major U.S. stock indices dropped nearly 9 percent and quickly rebounded.

High-frequency trading (HFT) is a type of algorithmic automated trading system in finance characterized by high speeds, high turnover rates, and high order-to-trade ratios that leverages high-frequency financial data and electronic trading tools. While there is no single definition of HFT, among its key attributes are highly sophisticated algorithms, co-location, and very short-term investment horizons in trading securities. HFT uses proprietary trading strategies carried out by computers to move in and out of positions in seconds or fractions of a second.

In finance, a stock index, or stock market index, is an index that measures the performance of a stock market, or of a subset of a stock market. It helps investors compare current stock price levels with past prices to calculate market performance.

The May 6, 2010, flash crash, also known as the crash of 2:45 or simply the flash crash, was a United States trillion-dollar flash crash which started at 2:32 p.m. EDT and lasted for approximately 36 minutes.