Tonic water is a carbonated beverage that contains water, sugar, and quinine, which gives it a distinct bitter flavor. Originally developed as a medicinal drink to prevent malaria, it is now commonly enjoyed as a mixer in cocktails, particularly with gin. The quinine in tonic water is derived from the bark of the cinchona tree, and it was historically used for its therapeutic properties. Today, tonic water is available in various brands and flavors, often with added citrus or herbal notes, making it a popular choice for refreshing drinks.