Monophthongization
The Century Dictionary and Cyclopedia
noun.
The reduction of a diphthong to a single sound.
Wiktionary, Creative Commons Attribution/Share-Alike License
noun.
The process of becoming
monophthong
, for a
diphthong
or
triphthong
.
Word Usage
"The second monophthongization is pretty much unique to Pittsburgh, at least among American English speakers."