In music, a ninth is a compound interval consisting of an octave plus a second. Wikipedia

Musical interval Wikidata

A compound musical interval consisting of an octave plus a second. OmegaWiki

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). Wiktionary