Crossword clue: a diacritical mark (two dots) placed over a vowel to indicate that it does not form a diphthong with an adjacent vowel

The most common answer is diaeresis (9 letters).

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

Possible answers

diaeresis 9 letters wn-def

a diacritical mark (two dots) placed over a vowel to indicate that it does not form a diphthong with an adjacent vowel

dieresis 8 letters wn-def

a diacritical mark (two dots) placed over a vowel to indicate that it does not form a diphthong with an adjacent vowel