noun the key tone, or first tone of any scale.
2. tonic ·noun a tonic element or letter; a vowel or a diphthong.
3. tonic ·noun a medicine that increases the strength, and gives vigor of action to the system.
4. tonic ·add. ·adj characterized by continuous muscular contraction; as, tonic convulsions.
5. tonic ·adj of or pertaining to tension; increasing tension; hence, increasing strength; as, tonic power.
6. tonic ·adj increasing strength, or the tone of the animal system; obviating the effects of debility, and restoring healthy functions.
7. tonic ·adj of or relating to tones or sounds; specifically (phon.), applied to, or distingshing, a speech sound made with tone unmixed and undimmed by obstruction, such sounds, namely, the vowels and diphthongs, being so called by dr. james rush (1833) " from their forming the purest and most plastic material of intonation.".