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.
      [1913 Webster]

   2. (Mus.) The key tone, or first tone of any scale.
      [1913 Webster]

   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.
      [1913 Webster]

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.
      [1913 Webster]

   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]

WordNet (r) 3.0 (2006):

tonic
    adj 1: of or relating to or producing normal tone or tonus in
           muscles or tissue; "a tonic reflex"; "tonic muscle
           contraction"
    2: employing variations in pitch to distinguish meanings of
       otherwise similar words; "Chinese is a tonal language" [syn:
       tonic, tonal]
    3: used of syllables; "a tonic syllables carries the main stress
       in a word" [syn: tonic, accented] [ant: atonic,
       unaccented]
    4: relating to or being the keynote of a major or minor scale;
       "tonic harmony"
    5: imparting vitality and energy; "the bracing mountain air"
       [syn: bracing, brisk, fresh, refreshing,
       refreshful, tonic]
    n 1: lime- or lemon-flavored carbonated water containing quinine
         [syn: tonic, tonic water, quinine water]
    2: a sweet drink containing carbonated water and flavoring; "in
       New England they call sodas tonics" [syn: pop, soda,
       soda pop, soda water, tonic]
    3: (music) the first note of a diatonic scale [syn: tonic,
       keynote]
    4: a medicine that strengthens and invigorates [syn: tonic,
       restorative]

Moby Thesaurus II by Grady Ward, 1.0:

209 Moby Thesaurus words for "tonic":
   accented, activating, activator, alcoholic drink, alveolar,
   analeptic, animating, animative, animator, apical, apico-alveolar,
   apico-dental, arouser, articulated, assimilated, astringent, back,
   barytone, beneficial, benign, beverage, bilabial, boost, bracer,
   bracing, brisk, broad, cacuminal, central, cerebral, checked,
   cheering, chromatic, close, consonant, consonantal, constitutional,
   continuant, cordial, corroborant, crisp, crispy, dental,
   dissimilated, dominant, dorsal, drink, drinkable, energizer,
   energizing, enharmonic, enlivening, enlivenment, exhilarating,
   exhilaration, exhilarative, flat, fortifying, fresh, front,
   frosted, frosted shake, glide, glossal, glottal, good, good for,
   guttural, hard, health-enhancing, health-preserving, healthful,
   healthy, heavy, high, human dynamo, hygeian, hygienic, intonated,
   invigorating, invigoration, invigorative, key, key signature,
   keynote, labial, labiodental, labiovelar, lateral, lax, life,
   light, lingual, liquid, liquor, low, major, major key, malt,
   mediant, mid, minor, monophthongal, motivating force, motive power,
   muted, narrow, nasal, nasalized, occlusive, open, oxytone, palatal,
   palatalized, pedal point, pharyngeal, pharyngealized, phonemic,
   phonetic, phonic, pick-me-up, pitch, pitched, pop, posttonic,
   potable, potation, quickening, reanimation, recreation, refection,
   refresher, refreshful, refreshing, refreshment, regale, regalement,
   regaling, reinvigoration, renewal, restorative, retroflex, revival,
   revivescence, revivescency, revivification, reviving, roborant,
   rounded, rousing, salubrious, salutary, sanitary, semitonic,
   semivowel, shake, sharp, soda, soda pop, soda water, soft,
   soft drink, sonant, spark plug, stimulant, stimulating,
   stimulation, stimulative, stimulator, stimulus, stopped,
   strengthening, stressed, strong, subdominant, submediant, subtonic,
   supertonic, surd, syllabic, tense, thick, throaty, tisane, tonal,
   tonality, tonic key, tonicity, tonus, twangy, unaccented,
   unrounded, unstressed, velar, viable, vitalizing, vitamin shot,
   vivification, vocalic, vocoid, voiced, voiceless, vowel, vowellike,
   weak, wholesome, wide, zestful, zesty