The Collaborative International Dictionary of English v.0.48:

Third \Third\, n.
   1. The quotient of a unit divided by three; one of three
      equal parts into which anything is divided.
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   2. The sixtieth part of a second of time.
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   3. (Mus.) The third tone of the scale; the mediant.
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   4. pl. (Law) The third part of the estate of a deceased
      husband, which, by some local laws, the widow is entitled
      to enjoy during her life.
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   Major third (Mus.), an interval of two tones.

   Minor third (Mus.), an interval of a tone and a half.
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