the mark ¨, in writing placed over the second of two adjacent vowels to indicate that it is to be pronounced separately rather than forming a diphthong with the first, as in some spellings of coöperate, naïve, etc
2.
this mark used for any other purpose, such as to indicate that a special pronunciation is appropriate to a particular vowel
Compare umlaut
3.
a pause in a line of verse occurring when the end of a foot coincides with the end of a word
Derived forms
diaeretic or dieretic (ˌdaɪəˈrɛtɪk)
adjective
Word origin
C17: from Latin diarēsis, from Greek diairesis a division, from diairein, from dia- + hairein to take; compare heresy