ninth

Concise Oxford English Dictionary © 2008 Oxford University Press:
ninth/nʌɪnθ/

▶ordinal number

• 1 constituting number nine in a sequence; 9th.

• 2 (a ninth/one ninth) each of nine equal parts into which something is or may be divided.

• 3 Music an interval spanning nine consecutive notes in a diatonic scale.

  ■ the note which is higher by this interval than the tonic of a diatonic scale or root of a chord.

– derivatives
ninthly adverb.