Tonic may mean:

• A concept from musical harmony and musical theory: see Tonic (music);
• A carbonated beverage flavoured with quinine, used in cocktails: see Tonic water.
• A carbonated soft drink ("tonic" is a regional expression in this sense, used primarily in and around Boston, Massachusetts, whereas other American locales prefer "soda", "pop", or "coke")
• A usually liquid drug given to improve health;
• A patent medicine that claims to have tonic properties;
• A band formed in the 1990s.
• A jazz club in New York City where Medeski Martin & Wood recorded and released their album, Tonic ^*.
• An American post-grunge band: see Tonic (band)