Quadruple-witching definitions

(finance) Simultaneous expiry on US markets of stock index futures, stock index options, stock options, and single stock futures, which takes place on the third Friday of March, June, September, and December. (Prior to single stock futures those days were triple witching.)

Origin of quadruple-witching

The idea of witching for these days is from the unusual patterns of trading that can occur ahead of the contract expiries.