A mark (¨) placed over a vowel to show that it should be pronounced separately from the vowel before it, as in 'naïve' or 'Zoë'.
From Greek 'dieresis' meaning 'division' or 'separation,' from 'dia-' (through) + 'hairesis' (taking). The mark indicates a hiatus—a break between two vowel sounds.
English speakers have been dropping diereses for decades (many style guides now say 'naive' instead of 'naïve'), which shows how language naturally simplifies even its own punctuation marks over time.
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