Synaeresis

In linguistics, synaeresis (/sɪˈnɛrəsɪs/; also spelled syneresis) is a phonological process of sound change in which two adjacent vowels within a word are combined into a single syllable.

The opposite process, in which two adjacent vowels are pronounced separately, is known as "diaeresis."

For any given word, speakers generally hold a traditional view about the standard pronunciation of that word. When realized in a careful reading style, each particular word is associated with this single, standard phonetic form. However, each word also possesses multiple non-standard or reduced phonetic forms which are produced in a greater range of contexts. These multiple variations in the pronunciation of a single word are referred to as allophonic variants. To classify one of these other forms as an allophonic variant of a word means that pronouncing the word in this way will not change the intended meaning of the word.

Synaeresis is one of various phonological processes in which segments of words or phrases are lost. The general term for a loss of sound segments in the field of linguistics is known as "elision." Other types of elision include the processes of apheresis, syncope, apocope, synizesis, and synaloepha.

Synaeresis comes from Greek συναίρεσις (synaíresis), a "contraction", a "taking or drawing together", from συναιρέω (synairéō), "(I) contract", "(I) grasp or seize together", derived from , "with", and , "(I) grasp, seize". Semantically, it is easy to understand how this term evolved historically. The term eventually becamer applied to a process in which vowels are taken or drawn together.

...
Wikipedia