adj. Pronounced as a single vowel sound; said especially of a vowel that is otherwise normally pronounced as a diphthong.
the GNU version of the Collaborative International Dictionary of English
adj. Consisting of, or pertaining to, a monophthong.
The Century Dictionary and Cyclopedia
Consisting of or pertaining to a monophthong.
Word Usage
"The modern monophthongal vowels come first, with the derived forms interspersed according to their form: first added i, then iotized, then iotized with added i."