Synaeresis in the context of "Diaeresis (prosody)"

In poetic meter, diaeresis (/daɪˈɛrəsɪs, -ˈɪər-/ dy-ERR-ə-siss, -⁠EER-; also spelled diæresis or dieresis) has two meanings: the separate pronunciation of the two vowels in a diphthong for the sake of meter, and a division between feet that corresponds to the division between words.

Synaeresis, the pronunciation of two vowels as a diphthong (or as a long vowel), is the opposite of the first definition.