Next in order or rank after the eighth, or before the tenth: an ordinal numeral: as, the ninth row; the ninth regiment.Being one of nine: as, a ninth part.n. A ninth part; the quotient of unity divided by nine.n. In music, the interval, whether melodic or harmonic, between any tone and a tone one octave and one degree distant from it; also, a tone distant by such an interval from a given tone; a compound second.