A monophthong (, ; Greek  from "single" and ? "sound") is a pure vowel sound, one whose articulation at both beginning and end is relatively fixed, and which does not glide up or down towards a new position of articulation. The monophthongs can be contrasted with diphthongs, where the vowel quality changes within the same syllable, and hiatus, where two vowels are next to each other in different syllables. A vowel sound whose quality does not change over the duration of the vowel is called a pure vowel.
The conversions of monophthongs to diphthongs (diphthongization), and of diphthongs to monophthongs (monophthongization), are major elements of language change and are likely the cause of further changes.
In some languages, due to monophthongization, graphemes that originally represented diphthongs now represent monophthongs.