Diaeresis \Di*[ae]r"e*sis\, Dieresis \Di*er"e*sis\ (?; 277),
         n.;
   pl. Di[ae]reses or
         Diereses. [L.
         diaeresis, Gr. ?, fr. ?
   to divide; dia`
         through, asunder + ? to take. See Heresy.]
   1. (Gram.)
         The separation or resolution of one syllable into
     
         two; -- the opposite of syn[ae]resis.
      [1913
         Webster]

   2. A mark consisting of two dots [[umlaut]],
         placed over the
      second of two adjacent vowels, to
         denote that they are to
      be pronounced as distinct
         letters; as, co["o]perate,
      a["e]rial.
      [1913
         Webster]