Which term refers to a combination of two vowels that glide into each other?

The correct term for a combination of two vowels that glide into each other is 'diphthong.' A diphthong occurs when two vowel sounds are combined within the same syllable, creating a smooth transition from one vowel sound to another. This phonetic phenomenon can be heard in words such as "coin" and "loud," where the sounds blend seamlessly and affect the way the word is pronounced.

The other terms have different meanings in the realm of linguistics. A digraph refers to a pair of letters that represent a single sound (such as "th" in "the"). A morpheme is the smallest grammatical unit in a language, which can be a word or a part of a word (like a prefix or suffix). A phoneme is the smallest unit of sound that can distinguish one word from another in a specific language. Understanding these distinctions helps clarify why 'diphthong' is the most suitable choice for describing the gliding combination of vowel sounds.