What is "triple witching hour"

n. The final hour of trading during which three types of derivatives contracts expire.

Triple witching hour is the last hour of the stock market trading session (3:00-4:00 P.M., New York City local Time) on the third Friday of every March, June, September, and December. Those days are the expiration of three kinds of securities:

• Stock market index futures;
• Stock market index options;
• Stock options.

The simultaneous expirations generally increases the trading volume of options, futures and the underlying stocks, and occasionally increases volatility of prices of related securities.

On those same days single-stock futures also expire, so that the final hour is sometimes referred to as the quadruple witching hour.