Triple sec is a clear orange flavored liqueur used in drink making as well as a flavoring for various desserts. It is one of the main ingredients in a margarita, a cosmopolitan a Long Island iced tea, and a whiskey sour. Other orange liqueurs include Grand Marnier, Cointreau, and Curacao. The main differences between all of these is typically price and quality.
This is typically easy to find in any liquor store or grocery store that sells alcohol. You can also find it online at Amazon.com: Triple Sec.
If you don't have triple sec you can use one of these alternations. We've included options that include no alcohol: