Di*aer"e*sis, Di*er"e*sis . [L. diaeresis, Gr. , fr. to divide; through, asunder + to take. See Heresy.]
The separation or resolution of one syllable into two; -- the opposite of synaeresis.
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.
© Webster 1913.