Vowel hiatus

In phonology, hiatus (/hˈtəs/ hy-AY-təs) or diaeresis (/dˈɛrəsɪs, -ˈɪər-/ dy-ERR-ə-siss, -EER-; also spelled dieresis or diæresis) describes the occurrence of two separate vowel sounds in adjacent syllables with no intervening consonant. When two vowel sounds instead occur together as part of a single syllable, the result is called a diphthong.