ETYM Cf. French tonigue, Greek. Related to Tone.
1. (Physiology) Of or relating to or producing normal tone or tonus in muscles or tissue.
2. Employing variations in pitch to distinguish meanings of otherwise similar words; SYN. tonal.
3. Relating to or being the keynote of a major or minor scale.
4. Used of syllables; SYN. accented.

ETYM Cf. French tonique, New Lat. tonicum.
1. Lime- or lemon-flavored carbonated water containing quinine; SYN. tonic water, quinine water.

Medicine, tonic medicine; Music, keynote.
A medicine that strengthens and invigorates; SYN. restorative.

4. Used of syllables; SYN. accented.

In music, the key note of a scale (for example, the note C in the scale of C major), or the “home key” in a composition (for example, the chord of C major in a composition in the same key).
(Music) The first note of a diatonic scale; SYN. keynote.