Source : Webster's Revised Unabridged Dictionary (1913)

Diatonic \Di`a*ton"ic\, a. [L. diatonicus, diatonus, Gr. ?, ?,
   fr. ? to stretch out; dia` through + ? to stretch: cf. F.
   diatonique. See {Tone}.] (Mus.)
   Pertaining to the scale of eight tones, the eighth of which
   is the octave of the first.

   {Diatonic scale} (Mus.), a scale consisting of eight sounds
      with seven intervals, of which two are semitones and five
      are whole tones; a modern major or minor scale, as
      distinguished from the {chromatic} scale.

Source : WordNet®

diatonic
     adj 1: based on the standard major or minor scales consisting of 5
            tones and 2 semitones without modulation by
            accidentals [ant: {chromatic}]
     2: based on or using the five tones and two semitones of the
        major or minor scales of Western music