The Collaborative International Dictionary of English v.0.48:

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]