a monophthong (pronounced "mono-f-thong") is simply a vowel. the word monophthong comes from the old greek language. "mono" means "one" or "single", and the "-phthong" means "sound" or "tone", from the basic word "phthalein", which means "to speak", "create sound with the voice". the word monophthong shows that a vowel is spoken with exactly one tone and one mouth position. for example, when you say "teeth", then while you are creating the sound of the "ee", nothing changes for that sound.