Diatonic means notes from a scale/key. Usually the well-used major. Does that mean any other notes not included are all chromatic? Would using notes from a parallel key change the definition? Definitions seem to have changed over time. And why can't both tags be included?!
'In all cases'? Now, that's just asking for trouble, particularly on an Internet forum! As you say, definitions have changed over time. And we could argue that the major scale is 'diatonic', the parallel minor contains chromatic alterations. Or that the natural minor is 'diatonic', the other minor scales are chromatically altered. Is 12-tone music 'chromatic' when it contains no contrasting diatonicism?
But you want an answer. A neat definition of what may be labelled 'chromatic'. Let's try this.
'A note not in the prevailing scale, or a chord using such notes'.
We don't need to worry WHERE the note came from - a 'parallel mode' or whatever. Ok, the note isn't chromatic in some OTHER scale. But it is chromatic in the one we're based in now.
I look at every note, chord, accidental, tone, no matter as being chromatic, I use non-diatonic to describe the chromatic tones used but not diatonic to a given scale or key, because the notes that are diatonic to any given scale or key are also included in the chromatic scale. I recognize that both terms are used somewhat interchangeably, but I feel it can actually lead to confusion for the student who is not yet accustomed to actual practices between musicians.