The Collaborative International Dictionary of English v.0.48:

Synaeresis \Syn*aer"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 diaeresis.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Syneresis \Syn*er"e*sis\, n.
   Same as Synaeresis.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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