Hiatus, in linguistics, refers to when one vowel succeeds another without an intervening consonant. The word comes from the Latin hio, hiare, meaning “to stand open, to gape”; the idea is that your mouth stays wide open, rather than closing, as it goes from one sound to the next.
In hiatus, each vowel constitutes a syllable or core of a syllable (in contrast to a diphthong or triphthong, in which the two or three vowel elements form a single syllable core).
Modern French dislikes or affects to dislike hiatus when it occurs between two words, preferring, whenever it can be managed, the pattern consonant-vowel-consonant-vowel (CVCV). This aversion is particularly evident in traditional rules of French prosody.
Two vowels succeeding one another within a word is a different story, and French verse allows them to remain a diphthong or be expanded (in what is called diérèse, “diaresis”) into two full syllables, according to the needs of the verse.