monophthong

noun

mon·​oph·​thong ˈmä-nə(f)-ˌthȯŋ 

: a vowel sound that throughout its duration has a single constant articulatory position

monophthongal

ˌmä-nə(f)-ˈthȯŋ-(g)əl 

adjective

Word History

Etymology

Late Greek monophthongos single vowel, from Greek mon- + phthongos sound

First Known Use

1616, in the meaning defined above