An example of a dipthong is the oa in the word coat.
Origin of diphthong
- Middle English diptonge from Old French diptongue from Late Latin diphthongus from Greek diphthongos di- two di–1 phthongos sound
From American Heritage Dictionary of the English Language, 5th Edition
- French diphtongue, from Ancient Greek δίφθογγος (diphthongos, “two sounds”), from δίς (dis, “twice”) + φθόγγος (phthongos, “sound”).