adjective

1. of, producing, or tending to produce good muscular tone, or tension
2. mentally or morally invigorating; stimulating
3. having to do with tones; specif.,
   1. Music designating or based on the first tone (keynote) of a diatonic scale a tonic chord
   2. Painting having to do with the tone or tones of a picture
   3. Phonet., Now Rare designating or of sounds characterized by resonance in the head cavities; also, accented
4. Med., Physiol. of or characterized by tone, or tonus

Etymology: Gr tonikos < tonos: see tone

noun

1. anything that invigorates or stimulates; specif.,
   1. a drug, medicine, or other agent for restoring or increasing body tone
   2. a hair or scalp dressing
2. 1. a carbonated beverage flavored with a little quinine and served in a mixed drink with gin, vodka, etc.; quinine water
   2. Chiefly Northeast soda pop
3. Music the first, or basic, tone of a diatonic scale; keynote
4. Phonet., Now Rare a tonic sound or syllable