Ninth meaning : The person or thing in the ninth position. One of nine equal parts of a whole. The compound interval between any tone and the tone represented on the ninth degree of the staff above it, as between one of the scale and two of the octave above; the octave of the second, consisting of 13 or 14 semitones (called minor and major ninth). To lose a ninth To divide by nine The ordinal form of the number nine.