diatonic

From French diatonique or Late Latin diatonicus, ultimately from Ancient Greek διατονικός (diatonikós), in the phrase [γένος (génos, “type, genus”)] διατονικός (diatonikós) (in reference to the diatonic tetrachord, and in contrast to the chromatic and enharmonic tetrachords), from διάτονος (diátonos) (διά (diá) + τόνος (tónos)), of disputed etymology, as both components are ambiguous.
Most plausibly, διάτονος (diátonos) refers to “stretched intervals”, as the intervals of the diatonic tetrachord are the most evenly distributed or “stretched out”, compared to the chromatic and enharmonic tetrads, which use smaller, more crowded together intervals. Compare pyknon, from πυκνός (puknós, “dense, compressed”), referring to the lower part of the non-diatonic tetrachords: the diatonic tetrachord has widely spaced notes (“stretched out”), while the other tetrachords have a closely spaced notes (“compressed”).