A dieresis is a punctuation mark that is placed over the second vowel of two adjacent vowels to indicate that they are not a sounded together, as in a dipthong. The two vowels are divided into two separate syllables. A dieresis consists of two dots. The word dieresis comes into the English language in the 1610s from the Greek word diairesis, which means division. Dieresis is the American spelling.
Diaeresis is the British spelling. A diaeresis looks similar to an umlaut and is often misidentified as such, but an umlaut is only used in the German language to signify the modification of the sound of a single vowel.
To begin with, English dictionaries generally treat “Aedes” as a three-syllable word, and they sometimes print it with a dieresis (“aëdes”) to indicate that. (The Washington Post)
But those at The New Yorker are something else entirely, a species nova that mutated into existence in 1925 and would hurl itself off a cliff rather than forsake the dieresis in “coöperate.” (The New York Times)
I’d be stuck in endless coöperation–or co-operation, should I try my hand at a piece for The Guardian–limbo before I could so much as look up the meaning of dieresis. (The Atlantic)
When Newbolt used it in his poem he did so as it was in Lucretius’, putting a diaeresis over the ‘i’ to show it was pronounced separately as Lucretius intended, but the diaeresis is rarely included in published forms of the work.” (The Sydney Morning Herald)
The chapters of Between You and Me are organized around the copy editor’s preoccupations: the comma, “Chances are that if you use the Oxford comma you brush the crumbs off your shirtfront before going out”; the hyphen, “The awful truth about hyphens and copy editors is that if there is one you want to take it out and if there’s not one you’re tempted to put one in”; the diaeresis, which Hobie Weekes, the magazine’s style editor contemplated eliminating, but he died and the issue has been left unaddressed since 1978; the pencil, with Ms. Norris’s paean to the Blackwing, “definitely softer than a No. 2, and very expressive.” (The Baltimore Sun)