The Collaborative International Dictionary of English v.0.48:

Major \Ma"jor\, [L. major, compar. of magnus great: cf. F.
   majeur. Cf. Master, Mayor, Magnitude, More, a.]
   1. Greater in number, quantity, or extent; as, the major part
      of the assembly; the major part of the revenue; the major
      part of the territory.
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   2. Of greater dignity; more important. --Shak.
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   3. Of full legal age; adult. [Obs.]
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   4. (Mus.) Greater by a semitone, either in interval or in
      difference of pitch from another tone.
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   Major key (Mus.), a key in which one and two, two and
      three, four and five, five and six and seven, make major
      seconds, and three and four, and seven and eight, make
      minor seconds.

   Major offense (Law), an offense of a greater degree which
      contains a lesser offense, as murder and robbery include
      assault.

   Major scale (Mus.), the natural diatonic scale, which has
      semitones between the third and fourth, and seventh and
      fourth, and seventh and eighth degrees; the scale of the
      major mode, of which the third is major. See Scale, and
      Diatonic.

   Major second (Mus.), a second between whose tones is a
      difference in pitch of a step.

   Major sixth (Mus.), a sixth of four steps and a half step.
      In major keys the third and sixth from the key tone are
      major. Major keys and intervals, as distinguished from
      minors, are more cheerful.

   Major third (Mus.), a third of two steps.
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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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