dieresis
/daɪˈɛrəsɪs/
noun
- A mark (¨) placed over a vowel to show that it is pronounced separately from the vowel before it, as in 'naïve' or 'coöperate'.
- The word 'naïve' has a dieresis over the 'i' to show it's pronounced as two syllables.
- Some writers use a dieresis in 'coöperate' to indicate the double 'o' is not a single sound.
- The dieresis is less common in modern English than it used to be.