Source : Webster's Revised Unabridged Dictionary (1913)

Synaeresis \Syn*[ae]r"e*sis\, Syneresis \Syn*er"e*sis\, n. [NL.,
   fr. Gr. ? a taking or drawing together, fr. ? to take
   together; sy`n with + ? to take, to grasp. See {Syn-}, and
   {Heresy}.] (Gram.)
   The union, or drawing together into one syllable, of two
   vowels that are ordinarily separated in syllabification;
   synecphonesis; -- the opposite of di[ae]resis.

Syneresis \Syn*er"e*sis\, n.
   Same as {Syn[ae]resis}.

Source : WordNet®

syneresis
     n 1: the contraction of two vowels into a diphthong [syn: {synaeresis}]
     2: the separation of liquid from a gel that is caused by
        contraction (as in cheese making) [syn: {synaeresis}]