How do you spell Dieresis

Available Definitions:
1)  n. - The separation or resolution of one syllable into two; -- the opposite of synaeresis.
2)  n. - 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, cooperate, aerial.
3)  n. - Same as Diaeresis.