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Tonic water, also known as Indian tonic water, is a soft drink, carbonated in nature with an additive called as Quinine, which is commonly used in combination with other drinks, like gin. Quinine is actually a medicine, which was used as a prophylactic agent against malaria, whose content has been reduced in tonic. Since it has medicinal effects due to addition of quinine, it got the name Tonic. Tonic water was intended to be used in South Asian and African regions, which has high cases of malaria.
Modern tonic water still has quinine, but the typical formulation is sweeter and more diluted. The purpose now is to help gin get down the throat more easily, so the tables have been turned. Tonic water is one of the few drinks that were once medicines that people now consume just for the purpose of taste. Quinine does have some side effects that make it questionable to use as a primary medicine, though.