A highly concentrated liquid made from distilling roots, plants, barks, herbs, etc. that is used to flavor cocktails, foods, or aperitifs. With a bitter or bittersweet taste, the stimulant is sometimes used for medicinal purposes. There are two types: aromatic bitters are made from the aforementioned roots and herbs, while flavored bitters are made from citrus fruits and the like.
When a cocktail recipe calls for Angostura bitters, Angostura refers to a brand rather than a specific type of bitters. Other well known brands are Fernet-Branca and Peychaud.