The Collaborative International Dictionary of English v.0.48:

Sixth \Sixth\, a. [From Six: cf. AS. sixta, siexta.]
   1. First after the fifth; next in order after the fifth.
      [1913 Webster]

   2. Constituting or being one of six equal parts into which
      anything is divided.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sixth \Sixth\, n.
   1. The quotient of a unit divided by six; one of six equal
      parts which form a whole.
      [1913 Webster]

   2. The next in order after the fifth.
      [1913 Webster]

   3. (Mus.) The interval embracing six diatonic degrees of the
      scale.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sixth
    adj 1: coming next after the fifth and just before the seventh
           in position [syn: sixth, 6th]
    n 1: position six in a countable series of things
    2: one part in six equal parts [syn: one-sixth, sixth]
    3: the musical interval between one note and another six notes
       away from it

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "sixth":
   consecutive intervals, decimalization, decimation, degree,
   diapason, diatessaron, diatonic interval, diatonic semitone,
   enharmonic diesis, enharmonic interval, fifth, fourth, half step,
   halftone, hexad, hexadic, hexagonal, hexahedral, hexamerous,
   hexangular, hexapartite, hexastyle, hexatomic, hexavalent,
   interval, less semitone, melodic interval, note, octave,
   parallel octaves, quinquepartition, quinquesection, second,
   semitone, senary, seventh, sexennial, sextipartite, sextipartition,
   sextuple, sixfold, step, tenth, third, tithe, tone,
   unison interval, whole step