Linguistics has suggested considerably different approaches with regard to diphthongs all of which can be summed up as follows: 1) A diphthong is the pronunciation of two vowels in one syllable (V1+V2) (N. Trubetzkoy, G. Trager, K. Pike, H. Gleason, A. Reformatsky, etc.) The following is yet another version of this definition – a diphthong is the pronunciation of syllabic sounds within a single syllable with only one component being syllabic (E. Aghayan). 2) A diphthong is the pronunciation of two vowels or a vowel and a semi-vowel (ṷ, i̭) in one syllable. In this case, it is not important whether the sonorant has the syllabic function of or not. (D. Jones, H. Acharyan, G. Ghapantsyan, etc.). 3) A diphthong is the combination of only a vowel and a (semi-vowel) sonorant (V+S / S+V) in one syllable (L. Bloomfield). Some distinguish between true and false diphthongs considering the combination of two vowels with equivalent components to belong to the first group, i.e., with no syllabic sound (L. Shcherba, J.Vahek, R. Budagov, etc.), unlike the three other types in which one of the diphthong components is syllabic (a nucleus of a diphthong). While these approaches regard a diphthong a minimum functional unit – a phoneme, another approach views a two-phoneme (= two-vowel) formation acting as one phonetic whole a diphthong. The components of a two-phoneme diphthong can be divided by a morphemic seam or can relate to adjacent syllables.
The key descriptions of diphthongs provide grounds for a broader understanding of the unit. Hence, a diphthong is the pronunciation of two vowels or a vowel and a syllabic sonorant as a phonetic whole, which phonemically can be equivalent to a complex phoneme uttered in the same syllable or to two phonemes divided by a morphemic seam and (or) two phonemes distributed between adjacent syllables. Based on this interpretation, diphthongs can be stable and non-stable: the former include two-vowel combinations uttered in the same syllable. The latter contain two-phoneme formations uttered in adjacent syllables and/or divided by a morphemic seam. According to the fullness of the sound, there are non-equivalent (with a syllabic component) and equivalent (without a syllabic component) diphthongs.
Following the given interpretation of the formation of diphthongs, the combination of a vowel and a non-syllabic (semi-vowel) sonorant in a single syllable is a diphthongoid rather than a diphthong.