From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
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