Triple sec is the orange-flavored liqueur traditionally used in a Cosmopolitan cocktail.
A standard Cosmopolitan combines vodka, triple sec, cranberry juice and freshly squeezed lime juice. The ingredients are usually shaken with ice and strained into a chilled cocktail glass, giving the drink its pink color.
Triple sec is a category of relatively dry orange liqueurs rather than one single brand. Cointreau is a common choice, and some recipes specify it by name, but “triple sec” is the traditional generic ingredient. This is why the orange component is sometimes described differently on menus.
The modern Cosmopolitan became especially prominent in the 1980s and 1990s. Its vivid color, citrus flavor and association with New York nightlife helped make it one of the best-known contemporary cocktails. It is distinct from a Cape Codder, which uses vodka and cranberry but does not traditionally include triple sec and lime in the same formula.