diaeresis

{ } n.; pl. Diæreses or Diereses [ L. diaeresis, Gr. &unr_;, fr. &unr_; to divide; dia` through, asunder + &unr_; to take. See Heresy. ] 1. (Gram.) The separation or resolution of one syllable into two; -- the opposite of synæresis. [ 1913 Webster ]

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. [ 1913 Webster ]