Wikipedia says that "diatonic" refers to a whole note scale or a scale with seven pitched per octave.
But my take is that the major and minor scale are built from five-note groupings with the intervals of WWHW and WHWW ... which is another way of saying that keys are built in fifths C-G-D-A etc. because C and G (I and V) are the most consonant notes ... the G is the 'secondary tonic'.
This is also the position of a music theory website ...
http://www.historyofmusictheory.com/?page_id=158 which states...
"This second tonic or “Di-tonic” which phonetically can be argued that this was the original meaning of “Dia-tonic” (Two tonics root and 5th producing the Hypo(dual tonic/di-tonic) scales as opposed to the common “Diatonic” naming convention."
But there is no historical reference here.
Is there an actual historical reference to our phonetic speculations that diatonic really means 'di-tonic'?