(I had to look it up.)
Knowing that the plural is diaereses probably means you have also no friends.
A diaeresis is the double dot you occasionally get above a letter, giving the instruction that the letter in question, and the one before it, should be pronounced separately, as in, for example, Noël,
Chloë and naïve.
There are also a few instances where a diaeresis tells you to pronounce the letter separately even though it isn't attached to another vowel, as in the name Brontë.
Diaereses used to be commoner in English - we used to write coöperate and poem, for example - but now if a word is likely to cause a problem with pronunciation then we tend to use a hyphen instead, as in, for example, the word re-ignite.
In fact, pronouncing adjacent vowels separately is still called a diaeresis, even without the double dots or hyphen - though only, as I've already said, by people who have wasted quite a substantial proportion of their lives.
Word Probably Not To Use Today: diaeresis. (Don't mix this word up with diuresis, which is to do with passing urine.) This word is Latin and comes from the Greek diairesis, which mean division. The Greek word diairein means to divide.
Diaeresis can also describe the place in a line of poetry when the end of a regular repeated rhythm coincides with the end of a word.
*Though some people spell it without the e, the rebels.