monophthong

A single vowel articulated without change in quality throughout the course of a syllable, as the vowel of English bed.

Two written vowels representing a single sound, as oa in boat.

A simple vowel-sound.

A combination of two written vowels pronounced as one.

A single uncompounded vowel sound.

A combination of two written vowels pronounced as one; a digraph.

A vowel (in the sense of a sound rather than a letter of the alphabet) that has the same sound throughout its pronunciation, such as the short vowels in “pap”, “pep”, “pip”, “pop” and “pup”, as opposed to a diphthong (eg, /aɪ/, the vowel in “pipe”) or a triphthong (eg, /aɪə/, the sound in the non-rhotic pronunciation of “pyre”).