( BrE ) ( US di·er·esis ) / daɪˈerəsɪs; NAmE / ( pl. di·aer·eses , di·er·eses / -siːz; NAmE /) noun
( technical ) the mark placed over a vowel to show that it is pronounced separately, as in naïve
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WORD ORIGIN
late 16th cent. (denoting the division of one syllable into two): via Latin from Greek diairesis separation, from diairein take apart, from dia apart + hairein take.