diphthongation

from The Century Dictionary.

• noun In philology, the formation of a diphthong; the conversion of a simple vowel into a diphthong by adding another vowel: as, Greek φαίν-ειν, from root *φαν; French rien, from Latin rem; Italian fuoco, from Latin focus, and the like.

from the GNU version of the Collaborative International Dictionary of English.

• noun See diphthongization.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun diphthongisation