/si ner"euh sis/ , n. Phonet.
1. the contraction of two syllables or two vowels into one, esp. the contraction of two vowels so as to form a diphthong.
2. synizesis.
Also, syneresis .
[ 1570-80; synaíresis act of taking together, equiv. to syn- SYN- + ( h)aire- (s. of haireîn to take) + -sis -SIS ]