Definition of diphthongize in English:

transitive verb

• Change (a vowel) into a diphthong.

  ‘the short vowel was not lengthened or diphthongized’

  More example sentences

  • ‘But at normal conversational speed its ghastly sequence of four diphthongized long vowels (in the nicely symmetrical but un-IPA Trager-Smith transcription, / ay ey iy ey /) sounds something like ah-ee-yay-ee-yee-yay-ee.’
  • ‘Stealth has a long, tense, diphthongized high vowel [i], whereas stealth has a short, lax, mid-vowel.’
  • ‘In particular, the vowel of the last syllable was a pretty good IPA [e], not diphthongized like the vowel in English babe but also not laxed and shortened like the vowel in English bed.’