ton·ic 
(tŏnĭk)
(tŏnĭk)n.
1.
a. An agent, such as a medication, that is supposed to restore or improve health or well-being.
b. A liquid preparation for the scalp or hair.
2. An invigorating, refreshing, or restorative agent or influence: Laughter was a tonic for the dispirited team.
3. See tonic water.
4. Boston See soft drink.
5. Music The first note of a diatonic scale; the keynote.
6. Linguistics A tonic accent.
adj.
1. Restorative or stimulating to health or well-being.
2.
a. Physiology Of, relating to, or producing tone or tonicity in muscles or tissue: a tonic reflex.
b. Medicine Characterized by continuous tension or contraction of muscles: a tonic convulsion or spasm.
3. Music Of or based on the keynote.
4. Stressed, as a syllable; accented.
[New Latin tonicus, of tension or tone, from Greek tonikos, capable of extension, from tonos, a stretching, tone; see TONE.]
toni·cal·ly adv.
Our Living Language Generic terms for carbonated soft drinks vary widely in the United States. Probably the two most common words competing for precedence are soda, used in the northeast United States as well as St. Louis and vicinity, and pop, used from the Midwest westward. In the South any soft drink, regardless of flavor or brand name, is referred to as a Coke, cold drink, or just plain drink. Speakers in Boston and its environs have a term of their own: tonic. See Note at dope.