Definition of diphthongize in English:

diphthongize

verb

[WITH OBJECT]
  • Change (a vowel) into a diphthong.

    • ‘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.’
    • ‘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.’

Pronunciation:

diphthongize

/ˈdifTHôNGˌīz//ˈdipTHôNGˌīz/