A diacritical mark (two dots) placed over a vowel in German to indicate a change in sound WordNet 3.0

A diacritical mark (two dots) placed over a vowel to indicate a change in sound in some languages Open English WordNet

The diaeresis is a diacritical mark used to indicate the separation of two distinct vowels in adjacent syllables when an instance of diaeresis occurs, so as to distinguish from a digraph or diphthong. Wikipedia

A diacritic consisting of two side-by-side dots that marks disyllabicity Wikipedia Disambiguation

A diacritic ( ¨ ) placed over a vowel letter (especially the second of two consecutive ones) indicating that it is sounded separately, usually forming a distinct syllable, as in the English words naïve, Noël and Brontë, the French haïr and the Dutch ruïne. Wiktionary

Diacritic placed over a vowel letter. Wiktionary (translation)

The diacritical mark ( ¨ ) placed over a vowel when it indicates a (rounded) front vowel. Wiktionary

Diacritical mark. Wiktionary (translation)