Ninth \Ninth\, a. [From Nine; cf. AS. nigo[eth]a.]

1. Following the eight and preceding the tenth; coming after eight others.

2. Constituting or being one of nine equal parts into which anything is divided.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Ninth \Ninth\, n.

1. The quotient of one divided by nine; one of nine equal parts of a thing; the next after the eighth.

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

Source: Webster's Revised Unabridged Dictionary (1913)