Webster's English Dictionary

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tonic

1. ton.ic \'ta:n-ik\ \'ta:n-i-k*-le-\ aj [Gk tonikos, fr. tonos tension, 
   tone] 1a: relating to or characterized by tension  1b: producing or adapted 
   to produce healthy muscular condition and reactio n 1c: characterized by 
   tonus {~ contraction of muscle}; also  : marked by prolonged muscular 
   contraction {~ convulsions} 2a: increasing or restoring physical or mental 
   tone : INVIGORATING  2b: yielding a tonic substance  3: relating to or 
   based on the first tone of a scale {~ harmony}  4a: VOICED  of a syllable  
   4b: bearing a principal stress or accent  5: of or relating to speech tones 
   or to languages using them to distinguis h words otherwise identical - 
   ton.i.cal.ly av
2. tonic n 1a: an agent (as a drug) that increases body tone  1b: something 
   that invigorates, restores, refreshes, or stimulates  1c: a liquid 
   preparation for the scalp  2: the first degree of a major or minor scale  
   3: a voiced sound