Noun
A single uncompounded vowel sound.
A combination of two written vowels pronounced as one; a digraph.
Source: Webster's dictionaryAn early change that separated Runic Danish from the other dialects of Old East Norse was the change of the diphthong æi to the monophthong é, as in stæinn to sténn "stone". Source: Internet
Vowel breaking or diphthongization is a vowel shift in which a monophthong becomes a diphthong. Source: Internet