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