The Collaborative International Dictionary of English v.0.48:

Tonic \Ton"ic\, a. [Cf. F. tonigue, Gr. ?. See Tone.]
   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."
      [1913 Webster]

   2. Of or pertaining to tension; increasing tension; hence,
      increasing strength; as, tonic power.
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   3. (Med.) Increasing strength, or the tone of the animal
      system; obviating the effects of debility, and restoring
      healthy functions.
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   4. (Med.) Characterized by continuous muscular contraction;
      as, tonic convulsions.
      [Webster 1913 Suppl.]

   Tonic spasm. (Med.) See the Note under Spasm.
      [1913 Webster]