A bitters is traditionally an alcoholic preparation flavored with botanical matter such that the end result is characterized by a bitter, sour, or bittersweet flavor. Numerous longstanding brands of bitters were originally developed as patent medicines, but are now sold as digestifs and cocktail flavorings.

The botanical ingredients used in preparing bitters have historically consisted of aromatic herbs, bark, roots, and/or fruit for their flavour and medicinal properties. Some of the more common ingredients are cascarilla, cassia, gentian, orange peel, and cinchona bark.

Most bitters contain both water and alcohol, the latter of which functions as a solvent for botanical extracts as well as a preservative. The alcoholic strength of bitters varies widely across different brands and styles.