Word: Diphthongise (also spelled "diphthongize")
Definition: The verb "diphthongise" means to change a simple vowel sound into a diphthong. A diphthong is a complex vowel sound that begins with one vowel sound and glides into another within the same syllable.
"Diphthongise" is a term used in linguistics to describe the process of changing a simple vowel sound into a more complex diphthong.