What is "monophthongization"

n. (context phonetics English) The process of becoming monophthong, for a diphthong or triphthong.

Monophthongization is a sound change by which a diphthong becomes a monophthong, a type of vowel shift. In languages that have undergone monophthongization, digraphs that formerly represented diphthongs now represent monophthongs. The opposite of monophthongization is vowel breaking.