WordNet: nonsyllabic, unsyllabic

not forming a syllable or the nucleus of a syllable; consisting of a consonant sound accompanied in the same syllable by a vowel sound or consisting of a vowel sound dominated by other vowel sounds in a syllable (as the second vowel in a falling diphthong); "the nonsyllabic `n' in `botany' when it is pronounced `botny'"; "the nonsyllabic `i' in `oi'"