Ton·ic a.
 1. 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.”
 2. Of or pertaining to tension; increasing tension; hence, increasing strength; as, tonic power.
 3. Med. Increasing strength, or the tone of the animal system; obviating the effects of debility, and restoring healthy functions.
 4. Med. Characterized by continuous muscular contraction; as, tonic convulsions.
 Tonic spasm. Med. See the Note under Spasm.