A diphthong is a complex vowel sound that begins with one vowel and glides into another within the same syllable. For example, in the English word "coin," the sound starts with the vowel o and moves to i. Diphthongs are important in many languages, as they can change the meaning of words and affect pronunciation. In English, common diphthongs include sounds like ai in "rain" and ou in "out." Understanding diphthongs helps with proper pronunciation and can improve language skills. They are a key feature in phonetics, the study of sounds in human speech.