Triple Witching
An event that occurs when the contracts for stock index futures, stock index options and stock options all expire on the same day. Triple witching days happen four times a year: the third Friday of March, June, September and December. This phenomenon is sometimes referred to as "Freaky Friday".
Investopedia Commentary
The final trading hour for that Friday is the hour known as triple witching. The markets are quite volatile in this final hour, as traders are quickly offsetting their option/futures orders before the closing bell. If you are a long-term investor, then triple witching has a minimal impact on you.
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See also: Closing Bell, Double Witching, Futures, Index, Index Futures, Index Option, Quadruple Witching, S&P 500, Stock Option, Volatility