a mark written over a vowel that shows it is pronounced separately from the vowel immediately before it, for example in the word naïve
分音符，隔音符（指「¨」，表示兩個母音須分別發音，如naïve）https://dictionary.cambridge.org/zht/%E8%A9%9E%E5%85%B8/%E8%8B%B1%E8%AA%9E-%E6%BC%A2%E8%AA%9E-%E7%B9%81%E9%AB%94/diaeresis
