Monophthong is a real word: here's the Webster's definition:

Main Entry: mon·oph·thong
Pronunciation: 'mä-n&(f)-"tho[ng]
Function: noun
Etymology: Late Greek monophthongos single vowel, from Greek mon- + phthongos sound
Date: 1616
: a vowel sound that throughout its duration has a single constant articulatory position
- mon·oph·thon·gal /"mä-n&(f)-'tho[ng]-(g)&l/ adjective

And let us not forget the magnificent triphthong:

Main Entry: triph·thong
Pronunciation: 'trif-"tho[ng], 'trip-
Function: noun
Etymology: tri- + -phthong (as in diphthong)
Date: circa 1599
1 : a phonological unit consisting of three successive vocalic sounds in one syllable
2 : TRIGRAPH
- triph·thon·gal /trif-'tho[ng]-g&l, trip-, -&l/ adjective


Triphthongs can be heard in some people's pronunciation of, e.g., our.


Bingley

Bingley