diaeresis
noun (British English)(US English dieresis)
(specialist)the mark placed over a vowel to show that it is pronounced separately, as in naïve 分音符（标在元音上面，表示单独发音，如 naïve） Word Originlate 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’.