To change a simple vowel sound into a diphthong, or to cause a sound to develop diphthongal qualities.
From 'diphthong' + '-alize' (to make or become). This verb form was created to describe the linguistic process where pure vowels gradually transform into gliding vowels over time.
English has been slowly diphthongizing its vowels for centuries—the 'ee' sound in 'fleece' used to be pure but is now becoming more of a glide, which is why accents vary so much around the world!
Complete word intelligence in one call. Free tier — 50 lookups/day.