1. Greek Music, That scale (the others being chromatic and enharmonic) in which the interval of a tone was used, the tetrachord being divided into two whole tones and a semitone (as in each half of the modern diatonic scale).
2. In modern music, denoting the scale which in any key proceeds by the notes proper to that key without chromatic alteration.

With apologies to Webster 1913, the Oxford Universal Dictionary on Historical Principles, third edition, has a more complete definition on this term.

Di`a*ton"ic (?), a. [L. diatonicus, diatonus, Gr. , , fr. to stretch out; 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.

© Webster 1913.