Definition from Wiktionary, the free dictionary
- A carbonated beverage containing quinine, which is a medicine, and is also used as a mixer in gin and tonic and other cocktails.
2009 Spring, Blue, Anthony Dias, “Mixology: Gin & Tonic”, in Drinks, the Magazine of Fine Wine, Spirits, and Living, page 8:
- Tonic water's active ingredient is quinine, a chemical compound derived from the bark of the cinchona tree, native to South America. This alkaloid has long been known to relieve the pain and fever of malaria and other tropical diseases, making it a staple "tonic" in warm-weather countries.