What is the technical term for two vowels sounded as one?

The technical term for two vowels that are pronounced as one sound is known as a diphthong. A diphthong occurs when a single syllable combines two adjacent vowel sounds in a way that they glide from one to the other, resulting in a smooth transition that creates a single cohesive sound.

For example, in the English word "coin," the sounds of the vowels 'o' and 'i' blend together, creating a diphthong. This is distinct from a monophthong, where a single, pure vowel sound is produced, and from a triphthong, which involves three vowel sounds in a single syllable. The term "vowel pair" is more informal and not recognized as a technical phonetic term, making it less accurate than diphthong in describing this particular phenomenon.