mon·oph·thong 
(mŏnəf-thông′, -thŏng′)
(mŏnəf-thông′, -thŏng′)n.
1. A single vowel articulated without change in quality throughout the course of a syllable, as the vowel of English bed.
2. Two written vowels representing a single sound, as oa in boat.
[Late Greek monophthongos : Greek mono-, mono- + Greek phthongos, sound.]
mon′oph·thongal (-thônggəl, -thŏng-) adj.