diaeresis
diacritic (U+0308) of two dots written horizontalement above a base letter, used to denote the separation of two consecutive vowels, or to replace the 2nd letter of a digraph modifying the pronunciation of the base letter
en-US/daɪˈɛɹɪsɪs/
en-GB/daɪˈɛɹəsɪs/