diaeresis vs diacritic

diaeresis and diacritic both are nouns.

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

As nouns, diacritic is a hypernym of diaeresis; that is, diacritic is a word with a broader meaning than diaeresis:

diaeresis: a diacritical mark (two dots) placed over a vowel to indicate that it does not form a diphthong with an adjacent vowel
diacritic: a mark added to a letter to indicate a special pronunciation

Other hypernyms of diaeresis include diacritical mark.

diaeresis (noun)	diacritic (noun)
a diacritical mark (two dots) placed over a vowel to indicate that it does not form a diphthong with an adjacent vowel	a mark added to a letter to indicate a special pronunciation

diaeresis (adjective)	diacritic (adjective)
	capable of distinguishing