What is a diphthong?

A diphthong is defined as a single but shifting sound created by the combination of two or more vowel sounds. This occurs when the articulatory position changes from one vowel sound to another within the same syllable, resulting in a glide between these sounds. For example, in the English words "coin" or "ow," the movement from one vowel sound to another is what characterizes the diphthong.

The other aspects of the question differentiate it from consonants, silent letters, or sounds that do not require a vowel. Therefore, understanding diphthongs involves recognizing them as unique vowel sounds that blend smoothly into each other, which is central to the correct answer.