spurious diphthong
Ancient Greek vowel that is etymologically a long vowel but written as if it were a true diphthong, i.e. as ⟨ει⟩, ⟨ου⟩
en-US/ˈspjʊɹiəs ˈdɪfˌθɔŋ/
en-GB/ˈspjʊɹɪəs ˈdɪfθɒŋ/
Ancient Greek vowel that is etymologically a long vowel but written as if it were a true diphthong, i.e. as ⟨ει⟩, ⟨ου⟩