Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

Source : WordNet®

diaeresis
     n : a diacritical mark (two dots) placed over a vowel in German
         to indicate a change in sound [syn: {umlaut}, {dieresis}]
     [also: {diaerses} (pl), {diaereses} (pl)]