[diph.thong] n [ME diptonge, fr. MF diptongue, fr. LL dipthongus, fr. Gk diphthongos, fr. di- + phthongos voice, sound] (15c) 1: a gliding monosyllabic speech sound (as the vowel combination at the end of toy) that starts at or near the articulatory position for one vowel and moves to or toward the position of another
2: digraph
3: the ligature a and e joined together or o and e joined together -- diph.thon.gal adj