What is "monophthong"

Monophthong \Mon"oph*thong\, n. [Gr. ? with one sound; mo`nos alone + ? sound, voice.]

1. A single uncompounded vowel sound.

2. A combination of two written vowels pronounced as one; a digraph.

n. A vowel (in the sense of a sound rather than a letter of the alphabet) that has the same sound throughout its pronunciation, such as the short vowels in "pap", "pep", "pip", "pop" and "pup", as opposed to a diphthong (eg, /aɪ/, the vowel in "pipe") or a triphthong (eg, /aɪə/, the sound in the non-rhotic pronunciation of "pyre").

A monophthong ( Greek monóphthongos from mónos "single" and phthóngos "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.