Witching hour is the last hour of stock market while trading is on between 3pm (when the bond market closes) and 4pm . Witching hour is typically controlled by large professional traders, program traders and large institutional traders, and can be characterized by higher-than-average volatility.
The witching hour is most commonly known in the context of triple witching, which is the third Friday of every quarter, when stock index options, stock options and stock index futures expire and roll to the next series. The last hour of these Fridays can be very volatile as positions are adjusted or closed out in anticipation of expiration. Since single stock index options now expire on the same day, triple witching and quadruple witching are used somewhat interchangeably.