synizesis
noun
syn·i·ze·sis ˌsi-nə-ˈzē-səs 
: contraction of two syllables into one by uniting in pronunciation two adjacent vowels
Word History
Etymology
Late Latin, from Greek synizēsis, from synizein to sit down together, collapse, blend, from syn- + hizein to sit down; akin to Latin sidere to sit down — more at subside
First Known Use
1846, in the meaning defined above