The Collaborative International Dictionary of English v.0.48:

Diatonic \Di`a*ton"ic\ (d[imac]`[.a]*t[o^]n"[i^]k), 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.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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