diaeresis vs dieresis

diaeresis and dieresis both are nouns.

Word	Noun	Adjective	Verb	Adverb
diaeresis	Yes	No	No	No
dieresis	Yes	No	No	No

As nouns, diaeresis and dieresis are synonyms defined as:

diaeresis and dieresis: a diacritical mark (two dots) placed over a vowel to indicate that it does not form a diphthong with an adjacent vowel

diaeresis (noun)	dieresis (noun)
a diacritical mark (two dots) placed over a vowel to indicate that it does not form a diphthong with an adjacent vowel	a diacritical mark (two dots) placed over a vowel to indicate that it does not form a diphthong with an adjacent vowel