Trading Volume

The quantity of shares, bonds or contracts traded during a designated time period for a security or an entire exchange.

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

Stock 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.