Amaretto
Amaretto is a sweet liqueur from Italy that is made with the pits of several different kinds of fruits, such as peaches and apricots. The pits impart a sour/bitter flavor that is complemented with the addition of sugar. The result tastes somewhat like almonds and is used in cooking and a variety of drinks.
Amaretto is produced in the United States as well. I urge you to use the real thing from Italy. The brands in the United States are often candy syrup sweet and lack any of the subtle bitterness that is present in the Italian versions. Also, the Italian versions are at a higher proof, and are less sweet.