Cointreau may just be the most famous orange liqueur in the world. Stylistically, it's considered a triple sec—a variety made with both bitter and sweet orange peels, as opposed to curaçao, which is made only with bitter orange peels. Cointreau is popular in many classic cocktails, like the Margarita, Sidecar, Corpse Reviver #2, and Cosmopolitan, but can also be enjoyed on its own as an aperitif or digestif.
Cointreau was invented in the mid-1800s by brothers Adolphe and Edouard-Jean Cointreau, and the first bottles were sold in 1875. In the century and a half since, it has cemented its place as one of the most critically-acclaimed liqueurs of all time.