Ninth

In music or music theory a ninth is the note nine scale degrees from the root of chord and also the interval between the root and the ninth.

A ninth is a compound second, but the ninth is considered less dissonant than the second.

A ninth chord is a chord with a ninth, and is an extended chord. Added tone chords with what might be called an added second are usually said to have an added ninth, two fifths up from the root.