Diaresis
Diaresis (?; 277) , noun
[Latin diaeresis, Greek diai`resis, from diaire`w to divide; dia` through, asunder + a'ire`w to take. See Heresy.]
1.
(Grammar) The separation or resolution of one syllable into two; -- the opposite of synaresis.
2.
A mark consisting of two dots [u], placed over the second of two adjacent vowels, to denote that they are to be pronounced as distinct letters; as, cooperate, aerial.
Also: Dieresis, Diaeresis