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Webster 1913 Edition

Adj.

Consisting of, or pertaining to, a monophthong.

Adj.

Consisting of a simple vowel-sound.

monophthongal ‎(not comparable)

(linguistics) Of or pertaining to a monophthong.
(linguistics, of a vowel) Pronounced as a single vowel sound; said especially of a vowel that is otherwise normally pronounced as a diphthong.

of or pertaining to a monophthong

pronounced as a single vowel sound