diphthong
Pronunciation: /ˈdɪfθɒŋ, ˈdɪp-/
noun
- a sound formed by the combination of two vowels in a single syllable, in which the sound begins as one vowel and moves towards another (as in coin, loud, and side). Often contrasted with monophthong, triphthong
- a digraph representing the sound of a diphthong or single vowel (as in feat).
- a compound vowel character; a ligature (such as æ).

Origin:
late Middle English: from French diphtongue, via late Latin from Greek diphthongos, from di- 'twice' + phthongos 'voice, sound'

For a discussion of the pronunciation of diphthong, see diphtheria (usage).