Synaeresis
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In linguistics, synaeresis (Greek συναίρεσις) is the contraction of two vowels into a diphthong (or long vowel). If synaeresis is used against convention, it may serve as a rhetorical figure (a metaplasm). The opposite phenomenon is known as diaeresis.
In historical linguistics, such contractions often result after the regular loss of a consonant that used to separate the two vowels. In this case, the resulting vowel or diphthong will typically carry a circumflex intonation.