Wiktionary
n. A carbonated beverage containing quinine, which is a medicine, and is also used as a mixer in gin and tonic and other cocktails.
WordNet
n. lime- or lemon-flavored carbonated water containing quinine [syn: tonic, quinine water]
Wikipedia
Tonic water (or Indian tonic water) is a carbonated soft drink in which quinine is dissolved. Originally used as a prophylactic against malaria, tonic water usually now has a significantly lower quinine content and is consumed for its distinctive bitter flavour. It is often used in mixed drinks, particularly in gin and tonic.
Usage examples of "tonic water".
A wooden crate served as a side table, holding Tony's pipe, a box of wooden matches, a glass, a bottle of gin, and a bottle of tonic water.
From where they sat he was drinking a harmless lemonade or tonic water.
Inside we found a shriveled lemon, two sticks of butter, a block of moldy Havarti cheese, condiments, and a bottle of tonic water.
But Alphonse is clearly so happy and Vivian is just hooting away and Sexton is locked in conversation with Sadie and McDermott has returned with two glasses and a bottle of tonic water and seems to be making drinks for both of them.
She filled glasses with ice cubes and lemon slices, then poured gin over them, hearing the explosion of the ice, leaving hardly any room for tonic water.
She filled two-thirds of a glass with ice and tonic water, then took a green bottle of Tanqueray from the pantry and filled the glass to the top.