The Collaborative International Dictionary of English v.0.48:

Ninth \Ninth\, a. [From Nine; cf. AS. nigo[eth]a.]
   1. Following the eight and preceding the tenth; coming after
      eight others.
      [1913 Webster]

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

The Collaborative International Dictionary of English v.0.48:

Ninth \Ninth\, n.
   1. The quotient of one divided by nine; one of nine equal
      parts of a thing; the next after the eighth.
      [1913 Webster]

   2. (Mus.)
      (a) An interval containing an octave and a second.
      (b) A chord of the dominant seventh with the ninth added.
          [1913 Webster]

WordNet (r) 3.0 (2006):

ninth
    adj 1: coming next after the eighth and just before the tenth in
           position [syn: ninth, 9th]
    n 1: position nine in a countable series of things; "going into
         the ninth they were a run ahead"
    2: one part in nine equal parts [syn: one-ninth, ninth]