Monophthongs
A vowel sound that throughout its duration has a single constant articulatory position;
vowel sounds made with a single tongue and lip position throughout their production.
Merriam-Webster. (n.d.). Monophthong. In Merriam-Webster.com dictionary. Retrieved January 30, 2024, from https://www.merriam-webster.com/dictionary/monophthong
Crystal, K. (2011). The Cambridge encyclopedia of the English language (2nd ed.). Cambridge University Press. (p. 272).