What is the meaning of Monophthong?

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").

Source: wiktionary.org