The Collaborative International Dictionary of English v.0.48:

Tonic \Ton"ic\, n. [Cf. F. tonique, NL. tonicum.]
   1. (Phon.) A tonic element or letter; a vowel or a diphthong.
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   2. (Mus.) The key tone, or first tone of any scale.
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   3. (Med.) A medicine that increases the strength, and gives
      vigor of action to the system.
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   Tonic sol-fa (Mus.), the name of the most popular among
      letter systems of notation (at least in England), based on
      key relationship, and hence called "tonic." Instead of the
      five lines, clefs, signature, etc., of the usual notation,
      it employs letters and the syllables do, re, mi, etc.,
      variously modified, with other simple signs of duration,
      of upper or lower octave, etc. See Sol-fa.
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