Trying to do my own homework here! I've read that only the standard major scale is diatonic. Also, I've read that the harmonic and melodic minor scales are also diatonic. Also I've read that the notes from any scale can be called diatonic to that scale. I've always gone with the first two definitions, but I've been wrong before! With such conflicting views, is there a definitive definition?
Diatonic, in music, any stepwise arrangement of the seven “natural” pitches (scale degrees) forming an octave without altering the established pattern of a key or mode—in particular, the major and natural minor scales.
I emphasized the in particular to show that it doesn't necessarily apply to just the major and natural minor scales.
The way I understood it, diatonic described a note's relationship to a scale/chord. In other words, C# is not a diatonic note in the G major scale, but D is; F# is diatonic in the C Lydian mode, but G# is not.
A common definition for the Diatonic Scale is that of the Major Scale or the Natural Minor Scale. However, the word diatonic is often used generically and is not as strictly defined as, say, the Major Scale.
So, for instance, many will refer to any mode of the Major Scale as being diatonic. You will also see many theorists referring to the various triads (and other chords) built from the Major Scale as being diatonic.
So, in short, the term Major Scale is more specific and strictly defined than the more generic word Diatonic (although the term Diatonic Scale almost always means, or implies, the Major Scale).
The definitive definition would probably be based on the scale the name implies, and its historical origin, from the Ancient Greek and probably pythagorean diatonic scale.
The confusion may come from the colloquial use of the term, which includes using Major scale as a base, or includes scale modifications such as Harmonic Minor.
From what I remember, my college theory class defined the diatonic scale as specifically comprised of a pattern of whole tones and semitones, with the semitones fully separated.
The Harmonic Minor scale used in a melodic context (playing the augmented second jump from the minor 6th to the major 7th in the melodic line) would not be considered true diatonic, but a modified scale. Used Harmonically where the raised 7th degree of the scale is used in context and as part of a Dominant chord and not used as a leading tone to the tonic, then the Harmonic scale could be described as diatonic.
To quote Grove's Dictionary of Music: "Diatonic (from Gk. dia tonos : ‘proceeding by whole tones’ ) Based on or derivable from an octave of seven notes in a particular configuration, as opposed to Chromatic and other forms of Scale . A seven-note scale is said to be diatonic when its octave span is filled by five tones and two semitones, with the semitones maximally separated, for example the major scale (T–T–S–T–T–T–S). The natural minor scale and the church modes ( see Mode ) are also diatonic."
So, specifically five tones and two semitones. At least, that's how I learned it.