The Probert Encyclopaedia of Music

DIATONIC

In music, the term diatonic was originally applied by the Greeks to one of their three genera of music. In modern music it is applied to the natural scale, and to the intervals, chorda, melodies, or harmony characteristic of it. A diatonic chord is a chord having no note chromatically altered. A diatonic interval is an interval formed by two notes of the diatonic scale unaltered by accidentals. A diatonic melody is a melody composed of notes belonging to one scale only.
Research Diatonic