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.