According to my trusty 'bible' - aka 'Oxford Companion to Music' -
Diatonic scales are the major and minor, made up of tones and semitones, (inc. +2 in the harmonic minor) as distinct from the chromatic scales, made entirely of semitones. Thus the modes are also diatonic.
Diatonic passages, intervals, chords and harmonies, all constructed from the notes of the prevailing key are, whilst chromatic passages, intervals, chords and harmonies are not.
Some authorities won't include harmonic and melodic minor notes, and define the intervals of diatonic scales to be the well known TTSTTTS.
I have a feeling there's not one correct answer...