Which term describes a long vowel that consists of two sounds?

The term that describes a long vowel consisting of two sounds is a diphthong. Diphthongs occur when two vowel sounds are combined within the same syllable, creating a complex vowel sound that transitions smoothly from one sound to another. This occurs, for example, in words like "coin" and "loud," where the pronunciation involves a shift from one vowel quality to another.

In contrast, discourse refers to spoken or written communication and doesn't relate to vowel sounds. Ellipsis involves the omission of words in a phrase or sentence but does not pertain to vowel sounds either. Elevation, on the other hand, usually pertains to a change in status or tone and likewise does not connect to the concept of vowel sounds directly. Thus, diphthong is the only term that accurately defines the characteristic of a long vowel made up of two sounds.