dieresis
Di*er"e*sis, n. Etym: [NL.] Defn: Same as Diæresis. Di*ær"e*sis, Di*er"e*sis, n.; pl. Diæreses or Diereses. Etym: [L. diaeresis, Gr. Heresy.] 1. (Gram.) Defn: The separation or resolution of one syllable into two; -- the opposite of synæresis. 2. A mark consisting of two dots [..], placed over the second of two adjacent vowels, to denote that they are to be pronounced as distinct letters; as, coöperate, aërial.