monophthongization
/ˌmɒnə(f)θɒŋɡʌɪˈzeɪʃn/(also monophthongisation)
noun
The process by which a vowel sound changes, or is changed, to a monophthong; an instance or result of this.
Origin
Late 19th century; earliest use found in Henry Sweet (1845–1912), phonetician and comparative philologist. From monophthong + -ization, after diphthongization.