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.
      [1913 Webster]

   2. Of greater dignity; more important. --Shak.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

major key
    n 1: a key whose harmony is based on the major scale [syn:
         major key, major mode]