Definition - A diacritic placed over a vowel to indicate that the vowel should be pronounced separately from the preceding letter.

Example -
In the word coöperate, the diacritic ¨ indicates that the word has four syllables.

Etymology -
The word derives from the Greek diairein, to divide or separate (from dia, apart + hairein, to take).

Oxford English Dictionary -
The first citation for the word in this sense is from 1611:
"Diæresis is when two points ouer a vowell diuide it from another vowell, as bouë, queuë."
(Cotgr. Nnnn)

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